diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index ef22e9dfd73e6..7a75d6e925f8f 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -9206,7 +9206,7 @@ namespace ts {
                         }
                     }
                     const sourceTypes = some(constructorTypes, t => !!(t.flags & ~TypeFlags.Nullable)) ? constructorTypes : types; // TODO: GH#18217
-                    type = getUnionType(sourceTypes!, UnionReduction.Subtype);
+                    type = getUnionType(sourceTypes!);
                 }
             }
             const widened = getWidenedType(addOptionality(type, /*isProperty*/ false, definedInMethod && !definedInConstructor));
diff --git a/src/compiler/moduleNameResolver.ts b/src/compiler/moduleNameResolver.ts
index 874344ae321be..15022bd59f67f 100644
--- a/src/compiler/moduleNameResolver.ts
+++ b/src/compiler/moduleNameResolver.ts
@@ -69,7 +69,8 @@ namespace ts {
         JavaScript, /** '.js' or '.jsx' */
         Json,       /** '.json' */
         TSConfig,   /** '.json' with `tsconfig` used instead of `index` */
-        DtsOnly /** Only '.d.ts' */
+        DtsOnly,    /** Only '.d.ts' */
+        TsOnly,     /** '.[cm]tsx?' but not .d.ts variants */
     }
 
     interface PathAndPackageId {
@@ -1290,7 +1291,19 @@ namespace ts {
     export function nodeModuleNameResolver(moduleName: string, containingFile: string, compilerOptions: CompilerOptions, host: ModuleResolutionHost, cache?: ModuleResolutionCache, redirectedReference?: ResolvedProjectReference): ResolvedModuleWithFailedLookupLocations;
     /* @internal */ export function nodeModuleNameResolver(moduleName: string, containingFile: string, compilerOptions: CompilerOptions, host: ModuleResolutionHost, cache?: ModuleResolutionCache, redirectedReference?: ResolvedProjectReference, lookupConfig?: boolean): ResolvedModuleWithFailedLookupLocations; // eslint-disable-line @typescript-eslint/unified-signatures
     export function nodeModuleNameResolver(moduleName: string, containingFile: string, compilerOptions: CompilerOptions, host: ModuleResolutionHost, cache?: ModuleResolutionCache, redirectedReference?: ResolvedProjectReference, lookupConfig?: boolean): ResolvedModuleWithFailedLookupLocations {
-        return nodeModuleNameResolverWorker(NodeResolutionFeatures.None, moduleName, getDirectoryPath(containingFile), compilerOptions, host, cache, lookupConfig ? tsconfigExtensions : (compilerOptions.resolveJsonModule ? tsPlusJsonExtensions : tsExtensions), redirectedReference);
+        let extensions;
+        if (lookupConfig) {
+            extensions = tsconfigExtensions;
+        }
+        else if (compilerOptions.noDtsResolution) {
+            extensions = [Extensions.TsOnly];
+            if (compilerOptions.allowJs) extensions.push(Extensions.JavaScript);
+            if (compilerOptions.resolveJsonModule) extensions.push(Extensions.Json);
+        }
+        else {
+            extensions = compilerOptions.resolveJsonModule ? tsPlusJsonExtensions : tsExtensions;
+        }
+        return nodeModuleNameResolverWorker(NodeResolutionFeatures.None, moduleName, getDirectoryPath(containingFile), compilerOptions, host, cache, extensions, redirectedReference);
     }
 
     function nodeModuleNameResolverWorker(features: NodeResolutionFeatures, moduleName: string, containingDirectory: string, compilerOptions: CompilerOptions, host: ModuleResolutionHost, cache: ModuleResolutionCache | undefined, extensions: Extensions[], redirectedReference: ResolvedProjectReference | undefined): ResolvedModuleWithFailedLookupLocations {
@@ -1299,6 +1312,11 @@ namespace ts {
         const failedLookupLocations: string[] = [];
         // conditions are only used by the node12/nodenext resolver - there's no priority order in the list,
         //it's essentially a set (priority is determined by object insertion order in the object we look at).
+        const conditions = features & NodeResolutionFeatures.EsmMode ? ["node", "import", "types"] : ["node", "require", "types"];
+        if (compilerOptions.noDtsResolution) {
+            conditions.pop();
+        }
+
         const state: ModuleResolutionState = {
             compilerOptions,
             host,
@@ -1306,7 +1324,7 @@ namespace ts {
             failedLookupLocations,
             packageJsonInfoCache: cache,
             features,
-            conditions: features & NodeResolutionFeatures.EsmMode ? ["node", "import", "types"] : ["node", "require", "types"]
+            conditions,
         };
 
         const result = forEach(extensions, ext => tryResolve(ext));
@@ -1533,20 +1551,22 @@ namespace ts {
                     default: return tryExtension(Extension.Dts);
                 }
             case Extensions.TypeScript:
+            case Extensions.TsOnly:
+                const useDts = extensions === Extensions.TypeScript;
                 switch (originalExtension) {
                     case Extension.Mjs:
                     case Extension.Mts:
                     case Extension.Dmts:
-                        return tryExtension(Extension.Mts) || tryExtension(Extension.Dmts);
+                        return tryExtension(Extension.Mts) || (useDts ? tryExtension(Extension.Dmts) : undefined);
                     case Extension.Cjs:
                     case Extension.Cts:
                     case Extension.Dcts:
-                        return tryExtension(Extension.Cts) || tryExtension(Extension.Dcts);
+                        return tryExtension(Extension.Cts) || (useDts ? tryExtension(Extension.Dcts) : undefined);
                     case Extension.Json:
                         candidate += Extension.Json;
-                        return tryExtension(Extension.Dts);
+                        return useDts ? tryExtension(Extension.Dts) : undefined;
                     default:
-                        return tryExtension(Extension.Ts) || tryExtension(Extension.Tsx) || tryExtension(Extension.Dts);
+                        return tryExtension(Extension.Ts) || tryExtension(Extension.Tsx) || (useDts ? tryExtension(Extension.Dts) : undefined);
                 }
             case Extensions.JavaScript:
                 switch (originalExtension) {
@@ -1813,6 +1833,7 @@ namespace ts {
             switch (extensions) {
                 case Extensions.JavaScript:
                 case Extensions.Json:
+                case Extensions.TsOnly:
                     packageFile = readPackageJsonMainField(jsonContent, candidate, state);
                     break;
                 case Extensions.TypeScript:
@@ -1893,14 +1914,16 @@ namespace ts {
     function extensionIsOk(extensions: Extensions, extension: Extension): boolean {
         switch (extensions) {
             case Extensions.JavaScript:
-                return extension === Extension.Js || extension === Extension.Jsx;
+                return extension === Extension.Js || extension === Extension.Jsx || extension === Extension.Mjs || extension === Extension.Cjs;
             case Extensions.TSConfig:
             case Extensions.Json:
                 return extension === Extension.Json;
             case Extensions.TypeScript:
-                return extension === Extension.Ts || extension === Extension.Tsx || extension === Extension.Dts;
+                return extension === Extension.Ts || extension === Extension.Tsx || extension === Extension.Mts || extension === Extension.Cts || extension === Extension.Dts || extension === Extension.Dmts || extension === Extension.Dcts;
+            case Extensions.TsOnly:
+                return extension === Extension.Ts || extension === Extension.Tsx || extension === Extension.Mts || extension === Extension.Cts;
             case Extensions.DtsOnly:
-                return extension === Extension.Dts;
+                return extension === Extension.Dts || extension === Extension.Dmts || extension === Extension.Dcts;
         }
     }
 
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 0831da97a629c..3a88c1b5c3774 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -4668,8 +4668,10 @@ namespace ts {
     export type AnyImportOrRequire = AnyImportSyntax | VariableDeclarationInitializedTo<RequireOrImportCall>;
 
     /* @internal */
-    export type AnyImportOrRequireStatement = AnyImportSyntax | RequireVariableStatement;
+    export type AnyImportOrBareOrAccessedRequire = AnyImportSyntax | VariableDeclarationInitializedTo<RequireOrImportCall | AccessExpression>;
 
+    /* @internal */
+    export type AnyImportOrRequireStatement = AnyImportSyntax | RequireVariableStatement;
 
     /* @internal */
     export type AnyImportOrReExport = AnyImportSyntax | ExportDeclaration;
@@ -6178,6 +6180,8 @@ namespace ts {
         assumeChangesOnlyAffectDirectDependencies?: boolean;
         noLib?: boolean;
         noResolve?: boolean;
+        /*@internal*/
+        noDtsResolution?: boolean;
         noUncheckedIndexedAccess?: boolean;
         out?: string;
         outDir?: string;
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 6dbd1da2e70f4..d44df87c458a6 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -933,6 +933,10 @@ namespace ts {
         }
     }
 
+    export function isAnyImportOrBareOrAccessedRequire(node: Node): node is AnyImportOrBareOrAccessedRequire {
+        return isAnyImportSyntax(node) || isVariableDeclarationInitializedToBareOrAccessedRequire(node);
+    }
+
     export function isLateVisibilityPaintedStatement(node: Node): node is LateVisibilityPaintedStatement {
         switch (node.kind) {
             case SyntaxKind.ImportDeclaration:
@@ -2558,14 +2562,14 @@ namespace ts {
         return decl.kind === SyntaxKind.FunctionDeclaration || isVariableDeclaration(decl) && decl.initializer && isFunctionLike(decl.initializer);
     }
 
-    export function tryGetModuleSpecifierFromDeclaration(node: AnyImportOrRequire): string | undefined {
+    export function tryGetModuleSpecifierFromDeclaration(node: AnyImportOrBareOrAccessedRequire): StringLiteralLike | undefined {
         switch (node.kind) {
             case SyntaxKind.VariableDeclaration:
-                return node.initializer.arguments[0].text;
+                return findAncestor(node.initializer, (node): node is RequireOrImportCall => isRequireCall(node, /*requireStringLiteralLikeArgument*/ true))?.arguments[0];
             case SyntaxKind.ImportDeclaration:
-                return tryCast(node.moduleSpecifier, isStringLiteralLike)?.text;
+                return tryCast(node.moduleSpecifier, isStringLiteralLike);
             case SyntaxKind.ImportEqualsDeclaration:
-                return tryCast(tryCast(node.moduleReference, isExternalModuleReference)?.expression, isStringLiteralLike)?.text;
+                return tryCast(tryCast(node.moduleReference, isExternalModuleReference)?.expression, isStringLiteralLike);
             default:
                 Debug.assertNever(node);
         }
@@ -3131,21 +3135,31 @@ namespace ts {
     // export = <EntityNameExpression>
     // export default <EntityNameExpression>
     // module.exports = <EntityNameExpression>
-    // {<Identifier>}
-    // {name: <EntityNameExpression>}
+    // module.exports.x = <EntityNameExpression>
+    // const x = require("...")
+    // const { x } = require("...")
+    // const x = require("...").y
+    // const { x } = require("...").y
     export function isAliasSymbolDeclaration(node: Node): boolean {
-        return node.kind === SyntaxKind.ImportEqualsDeclaration ||
+        if (node.kind === SyntaxKind.ImportEqualsDeclaration ||
             node.kind === SyntaxKind.NamespaceExportDeclaration ||
             node.kind === SyntaxKind.ImportClause && !!(node as ImportClause).name ||
             node.kind === SyntaxKind.NamespaceImport ||
             node.kind === SyntaxKind.NamespaceExport ||
             node.kind === SyntaxKind.ImportSpecifier ||
             node.kind === SyntaxKind.ExportSpecifier ||
-            node.kind === SyntaxKind.ExportAssignment && exportAssignmentIsAlias(node as ExportAssignment) ||
+            node.kind === SyntaxKind.ExportAssignment && exportAssignmentIsAlias(node as ExportAssignment)
+        ) {
+            return true;
+        }
+
+        return isInJSFile(node) && (
             isBinaryExpression(node) && getAssignmentDeclarationKind(node) === AssignmentDeclarationKind.ModuleExports && exportAssignmentIsAlias(node) ||
-            isPropertyAccessExpression(node) && isBinaryExpression(node.parent) && node.parent.left === node && node.parent.operatorToken.kind === SyntaxKind.EqualsToken && isAliasableExpression(node.parent.right) ||
-            node.kind === SyntaxKind.ShorthandPropertyAssignment ||
-            node.kind === SyntaxKind.PropertyAssignment && isAliasableExpression((node as PropertyAssignment).initializer);
+            isPropertyAccessExpression(node)
+                && isBinaryExpression(node.parent)
+                && node.parent.left === node
+                && node.parent.operatorToken.kind === SyntaxKind.EqualsToken
+                && isAliasableExpression(node.parent.right));
     }
 
     export function getAliasDeclarationFromName(node: EntityName): Declaration | undefined {
@@ -3156,6 +3170,7 @@ namespace ts {
             case SyntaxKind.ExportSpecifier:
             case SyntaxKind.ExportAssignment:
             case SyntaxKind.ImportEqualsDeclaration:
+            case SyntaxKind.NamespaceExport:
                 return node.parent as Declaration;
             case SyntaxKind.QualifiedName:
                 do {
@@ -5142,6 +5157,11 @@ namespace ts {
             (node.parent.kind === SyntaxKind.PropertyAccessExpression && (node.parent as PropertyAccessExpression).name === node);
     }
 
+    export function isRightSideOfAccessExpression(node: Node) {
+        return isPropertyAccessExpression(node.parent) && node.parent.name === node
+            || isElementAccessExpression(node.parent) && node.parent.argumentExpression === node;
+    }
+
     export function isRightSideOfQualifiedNameOrPropertyAccessOrJSDocMemberName(node: Node) {
         return isQualifiedName(node.parent) && node.parent.right === node
             || isPropertyAccessExpression(node.parent) && node.parent.name === node
@@ -5829,6 +5849,45 @@ namespace ts {
         return expr;
     }
 
+    export function forEachNameInAccessChainWalkingLeft<T>(name: MemberName | StringLiteralLike, action: (name: MemberName | StringLiteralLike) => T | undefined): T | undefined {
+        if (isAccessExpression(name.parent) && isRightSideOfAccessExpression(name)) {
+            return walkAccessExpression(name.parent);
+        }
+
+        function walkAccessExpression(access: AccessExpression): T | undefined {
+            if (access.kind === SyntaxKind.PropertyAccessExpression) {
+                const res = action(access.name);
+                if (res !== undefined) {
+                    return res;
+                }
+            }
+            else if (access.kind === SyntaxKind.ElementAccessExpression) {
+                if (isIdentifier(access.argumentExpression) || isStringLiteralLike(access.argumentExpression)) {
+                    const res = action(access.argumentExpression);
+                    if (res !== undefined) {
+                        return res;
+                    }
+                }
+                else {
+                    // Chain interrupted by non-static-name access 'x[expr()].y.z'
+                    return undefined;
+                }
+            }
+
+            if (isAccessExpression(access.expression)) {
+                return walkAccessExpression(access.expression);
+            }
+            if (isIdentifier(access.expression)) {
+                // End of chain at Identifier 'x.y.z'
+                return action(access.expression);
+            }
+            // End of chain at non-Identifier 'x().y.z'
+            return undefined;
+        }
+    }
+
+
+
     export function getLeftmostExpression(node: Expression, stopAtCallExpressions: boolean) {
         while (true) {
             switch (node.kind) {
diff --git a/src/harness/client.ts b/src/harness/client.ts
index 296832388baf6..388f6473fa8da 100644
--- a/src/harness/client.ts
+++ b/src/harness/client.ts
@@ -298,7 +298,7 @@ namespace ts.server {
         getDefinitionAndBoundSpan(fileName: string, position: number): DefinitionInfoAndBoundSpan {
             const args: protocol.FileLocationRequestArgs = this.createFileLocationRequestArgs(fileName, position);
 
-            const request = this.processRequest<protocol.DefinitionRequest>(CommandNames.DefinitionAndBoundSpan, args);
+            const request = this.processRequest<protocol.DefinitionAndBoundSpanRequest>(CommandNames.DefinitionAndBoundSpan, args);
             const response = this.processResponse<protocol.DefinitionInfoAndBoundSpanResponse>(request);
             const body = Debug.checkDefined(response.body); // TODO: GH#18217
 
@@ -332,6 +332,23 @@ namespace ts.server {
             }));
         }
 
+        getSourceDefinitionAndBoundSpan(fileName: string, position: number): DefinitionInfo[] {
+            const args: protocol.FileLocationRequestArgs = this.createFileLocationRequestArgs(fileName, position);
+            const request = this.processRequest<protocol.FindSourceDefinitionRequest>(CommandNames.FindSourceDefinition, args);
+            const response = this.processResponse<protocol.DefinitionResponse>(request);
+            const body = Debug.checkDefined(response.body); // TODO: GH#18217
+
+            return body.map(entry => ({
+                containerKind: ScriptElementKind.unknown,
+                containerName: "",
+                fileName: entry.file,
+                textSpan: this.decodeSpan(entry),
+                kind: ScriptElementKind.unknown,
+                name: "",
+                unverified: entry.unverified,
+            }));
+        }
+
         getImplementationAtPosition(fileName: string, position: number): ImplementationLocation[] {
             const args = this.createFileLocationRequestArgs(fileName, position);
 
diff --git a/src/harness/fourslashImpl.ts b/src/harness/fourslashImpl.ts
index f4b2832c00b6e..d65aa86fb6b02 100644
--- a/src/harness/fourslashImpl.ts
+++ b/src/harness/fourslashImpl.ts
@@ -697,6 +697,13 @@ namespace FourSlash {
             this.verifyGoToX(arg0, endMarkerNames, () => this.getGoToDefinitionAndBoundSpan());
         }
 
+        public verifyGoToSourceDefinition(startMarkerNames: ArrayOrSingle<string>, end?: ArrayOrSingle<string | { marker: string, unverified?: boolean }> | { file: string, unverified?: boolean }) {
+            if (this.testType !== FourSlashTestType.Server) {
+                this.raiseError("goToSourceDefinition may only be used in fourslash/server tests.");
+            }
+            this.verifyGoToX(startMarkerNames, end, () => (this.languageService as ts.server.SessionClient).getSourceDefinitionAndBoundSpan(this.activeFile.fileName, this.currentCaretPosition)!);
+        }
+
         private getGoToDefinition(): readonly ts.DefinitionInfo[] {
             return this.languageService.getDefinitionAtPosition(this.activeFile.fileName, this.currentCaretPosition)!;
         }
@@ -764,7 +771,9 @@ namespace FourSlash {
             }
 
             if (endMarkers.length !== definitions.length) {
-                this.raiseError(`${testName} failed - expected to find ${endMarkers.length} definitions but got ${definitions.length}`);
+                const markers = definitions.map(d => ({ text: "HERE", fileName: d.fileName, position: d.textSpan.start }));
+                const actual = this.renderMarkers(markers);
+                this.raiseError(`${testName} failed - expected to find ${endMarkers.length} definitions but got ${definitions.length}\n\n${actual}`);
             }
 
             ts.zipWith(endMarkers, definitions, (endMarkerOrFileResult, definition, i) => {
@@ -774,17 +783,8 @@ namespace FourSlash {
                 ts.Debug.assert(typeof expectedFileName === "string");
                 const expectedPosition = marker?.position || 0;
                 if (ts.comparePaths(expectedFileName, definition.fileName, /*ignoreCase*/ true) !== ts.Comparison.EqualTo || expectedPosition !== definition.textSpan.start) {
-                    const filesToDisplay = ts.deduplicate([expectedFileName, definition.fileName], ts.equateValues);
                     const markers = [{ text: "EXPECTED", fileName: expectedFileName, position: expectedPosition }, { text: "ACTUAL", fileName: definition.fileName, position: definition.textSpan.start }];
-                    const text = filesToDisplay.map(fileName => {
-                        const markersToRender = markers.filter(m => m.fileName === fileName).sort((a, b) => b.position - a.position);
-                        let fileContent = this.tryGetFileContent(fileName) || "";
-                        for (const marker of markersToRender) {
-                            fileContent = fileContent.slice(0, marker.position) + `\x1b[1;4m/*${marker.text}*/\x1b[0;31m` + fileContent.slice(marker.position);
-                        }
-                        return `// @Filename: ${fileName}\n${fileContent}`;
-                    }).join("\n\n");
-
+                    const text = this.renderMarkers(markers);
                     this.raiseError(`${testName} failed for definition ${markerName || expectedFileName} (${i}): expected ${expectedFileName} at ${expectedPosition}, got ${definition.fileName} at ${definition.textSpan.start}\n\n${text}\n`);
                 }
                 if (definition.unverified && (typeof endMarkerOrFileResult === "string" || !endMarkerOrFileResult.unverified)) {
@@ -798,6 +798,18 @@ namespace FourSlash {
             });
         }
 
+        private renderMarkers(markers: { text: string, fileName: string, position: number }[]) {
+            const filesToDisplay = ts.deduplicate(markers.map(m => m.fileName), ts.equateValues);
+            return filesToDisplay.map(fileName => {
+                const markersToRender = markers.filter(m => m.fileName === fileName).sort((a, b) => b.position - a.position);
+                let fileContent = this.tryGetFileContent(fileName) || "";
+                for (const marker of markersToRender) {
+                    fileContent = fileContent.slice(0, marker.position) + `\x1b[1;4m/*${marker.text}*/\x1b[0;31m` + fileContent.slice(marker.position);
+                }
+                return `// @Filename: ${fileName}\n${fileContent}`;
+            }).join("\n\n");
+        }
+
         private verifyDefinitionTextSpan(defs: ts.DefinitionInfoAndBoundSpan, startMarkerName: string) {
             const range = this.testData.ranges.find(range => this.markerName(range.marker!) === startMarkerName);
 
diff --git a/src/harness/fourslashInterfaceImpl.ts b/src/harness/fourslashInterfaceImpl.ts
index ea07f4f386fc4..6eb4414df25ac 100644
--- a/src/harness/fourslashInterfaceImpl.ts
+++ b/src/harness/fourslashInterfaceImpl.ts
@@ -324,6 +324,10 @@ namespace FourSlashInterface {
             this.state.verifyGoToType(arg0, endMarkerName);
         }
 
+        public goToSourceDefinition(startMarkerNames: ArrayOrSingle<string>, end: { file: string } | ArrayOrSingle<string>) {
+            this.state.verifyGoToSourceDefinition(startMarkerNames, end);
+        }
+
         public goToDefinitionForMarkers(...markerNames: string[]) {
             this.state.verifyGoToDefinitionForMarkers(markerNames);
         }
diff --git a/src/server/editorServices.ts b/src/server/editorServices.ts
index 5a81aff9cc461..d40294989ee68 100644
--- a/src/server/editorServices.ts
+++ b/src/server/editorServices.ts
@@ -715,6 +715,8 @@ namespace ts.server {
         readonly newInferredProjectName = createProjectNameFactoryWithCounter(makeInferredProjectName);
         /*@internal*/
         readonly newAutoImportProviderProjectName = createProjectNameFactoryWithCounter(makeAutoImportProviderProjectName);
+        /*@internal*/
+        readonly newAuxiliaryProjectName = createProjectNameFactoryWithCounter(makeAuxiliaryProjectName);
         /**
          * Open files: with value being project root path, and key being Path of the file that is open
          */
@@ -989,13 +991,15 @@ namespace ts.server {
 
         private delayUpdateProjectGraph(project: Project) {
             project.markAsDirty();
-            const projectName = project.getProjectName();
-            this.pendingProjectUpdates.set(projectName, project);
-            this.throttledOperations.schedule(projectName, /*delay*/ 250, () => {
-                if (this.pendingProjectUpdates.delete(projectName)) {
-                    updateProjectIfDirty(project);
-                }
-            });
+            if (project.projectKind !== ProjectKind.AutoImportProvider && project.projectKind !== ProjectKind.Auxiliary) {
+                const projectName = project.getProjectName();
+                this.pendingProjectUpdates.set(projectName, project);
+                this.throttledOperations.schedule(projectName, /*delay*/ 250, () => {
+                    if (this.pendingProjectUpdates.delete(projectName)) {
+                        updateProjectIfDirty(project);
+                    }
+                });
+            }
         }
 
         /*@internal*/
@@ -2297,7 +2301,7 @@ namespace ts.server {
             configFileExistenceInfo.config.watchedDirectoriesStale = undefined;
         }
 
-        private updateNonInferredProjectFiles<T>(project: ExternalProject | ConfiguredProject | AutoImportProviderProject, files: T[], propertyReader: FilePropertyReader<T>) {
+        private updateNonInferredProjectFiles<T>(project: Project, files: T[], propertyReader: FilePropertyReader<T>) {
             const projectRootFilesMap = project.getRootFilesMap();
             const newRootScriptInfoMap = new Map<string, true>();
 
@@ -3589,7 +3593,7 @@ namespace ts.server {
             const toRemoveScriptInfos = new Map(this.filenameToScriptInfo);
             this.filenameToScriptInfo.forEach(info => {
                 // If script info is open or orphan, retain it and its dependencies
-                if (!info.isScriptOpen() && info.isOrphan() && !info.isContainedByAutoImportProvider()) {
+                if (!info.isScriptOpen() && info.isOrphan() && !info.isContainedByBackgroundProject()) {
                     // Otherwise if there is any source info that is alive, this alive too
                     if (!info.sourceMapFilePath) return;
                     let sourceInfos: Set<Path> | undefined;
diff --git a/src/server/project.ts b/src/server/project.ts
index 715a659281788..f4a4f51b46192 100644
--- a/src/server/project.ts
+++ b/src/server/project.ts
@@ -5,6 +5,7 @@ namespace ts.server {
         Configured,
         External,
         AutoImportProvider,
+        Auxiliary,
     }
 
     /* @internal */
@@ -209,6 +210,9 @@ namespace ts.server {
         /*@internal*/
         private packageJsonsForAutoImport: Set<string> | undefined;
 
+        /*@internal*/
+        private noDtsResolutionProject?: AuxiliaryProject | undefined;
+
         /*@internal*/
         getResolvedProjectReferenceToRedirect(_fileName: string): ResolvedProjectReference | undefined {
             return undefined;
@@ -811,6 +815,10 @@ namespace ts.server {
                 this.autoImportProviderHost.close();
             }
             this.autoImportProviderHost = undefined;
+            if (this.noDtsResolutionProject) {
+                this.noDtsResolutionProject.close();
+            }
+            this.noDtsResolutionProject = undefined;
 
             // signal language service to release source files acquired from document registry
             this.languageService.dispose();
@@ -1406,6 +1414,7 @@ namespace ts.server {
                 const oldOptions = this.compilerOptions;
                 this.compilerOptions = compilerOptions;
                 this.setInternalCompilerOptionsForEmittingJsFiles();
+                this.noDtsResolutionProject?.setCompilerOptions(this.getCompilerOptionsForNoDtsResolutionProject());
                 if (changesAffectModuleResolution(oldOptions, compilerOptions)) {
                     // reset cached unresolved imports if changes in compiler options affected module resolution
                     this.cachedUnresolvedImportsPerFile.clear();
@@ -1780,6 +1789,54 @@ namespace ts.server {
         getIncompleteCompletionsCache() {
             return this.projectService.getIncompleteCompletionsCache();
         }
+
+        /*@internal*/
+        getNoDtsResolutionProject(rootFileNames: readonly string[]): Project {
+            Debug.assert(this.projectService.serverMode === LanguageServiceMode.Semantic);
+            if (!this.noDtsResolutionProject) {
+                this.noDtsResolutionProject = new AuxiliaryProject(this.projectService, this.documentRegistry, this.getCompilerOptionsForNoDtsResolutionProject());
+            }
+
+            enumerateInsertsAndDeletes<NormalizedPath, NormalizedPath>(
+                rootFileNames.map(toNormalizedPath),
+                this.noDtsResolutionProject.getRootFiles(),
+                getStringComparer(!this.useCaseSensitiveFileNames()),
+                pathToAdd => {
+                    const info = this.projectService.getOrCreateScriptInfoNotOpenedByClient(
+                        pathToAdd,
+                        this.currentDirectory,
+                        this.noDtsResolutionProject!.directoryStructureHost);
+                    if (info) {
+                        this.noDtsResolutionProject!.addRoot(info, pathToAdd);
+                    }
+                },
+                pathToRemove => {
+                    // It may be preferable to remove roots only once project grows to a certain size?
+                    const info = this.noDtsResolutionProject!.getScriptInfo(pathToRemove);
+                    if (info) {
+                        this.noDtsResolutionProject!.removeRoot(info);
+                    }
+                },
+            );
+
+            return this.noDtsResolutionProject;
+        }
+
+        /*@internal*/
+        private getCompilerOptionsForNoDtsResolutionProject() {
+            return {
+                ...this.getCompilerOptions(),
+                noDtsResolution: true,
+                allowJs: true,
+                maxNodeModuleJsDepth: 3,
+                diagnostics: false,
+                skipLibCheck: true,
+                sourceMap: false,
+                types: ts.emptyArray,
+                lib: ts.emptyArray,
+                noLib: true,
+            };
+        }
     }
 
     function getUnresolvedImports(program: Program, cachedUnresolvedImportsPerFile: ESMap<Path, readonly string[]>): SortedReadonlyArray<string> {
@@ -1920,6 +1977,32 @@ namespace ts.server {
         }
     }
 
+    class AuxiliaryProject extends Project {
+        constructor(projectService: ProjectService, documentRegistry: DocumentRegistry, compilerOptions: CompilerOptions) {
+            super(projectService.newAuxiliaryProjectName(),
+                ProjectKind.Auxiliary,
+                projectService,
+                documentRegistry,
+                /*hasExplicitListOfFiles*/ false,
+                /*lastFileExceededProgramSize*/ undefined,
+                compilerOptions,
+                /*compileOnSaveEnabled*/ false,
+                /*watchOptions*/ undefined,
+                projectService.host,
+                /*currentDirectory*/ undefined);
+        }
+
+        isOrphan(): boolean {
+            return true;
+        }
+
+        /*@internal*/
+        scheduleInvalidateResolutionsOfFailedLookupLocations(): void {
+            // Invalidation will happen on-demand as part of updateGraph
+            return;
+        }
+    }
+
     export class AutoImportProviderProject extends Project {
         /*@internal*/
         private static readonly maxDependencies = 10;
@@ -2129,6 +2212,12 @@ namespace ts.server {
             return hasSameSetOfFiles;
         }
 
+        /*@internal*/
+        scheduleInvalidateResolutionsOfFailedLookupLocations(): void {
+            // Invalidation will happen on-demand as part of updateGraph
+            return;
+        }
+
         hasRoots() {
             return !!this.rootFileNames?.length;
         }
diff --git a/src/server/protocol.ts b/src/server/protocol.ts
index df61ead170eb4..fc53ad151ed67 100644
--- a/src/server/protocol.ts
+++ b/src/server/protocol.ts
@@ -83,6 +83,7 @@ namespace ts.server.protocol {
         SignatureHelp = "signatureHelp",
         /* @internal */
         SignatureHelpFull = "signatureHelp-full",
+        FindSourceDefinition = "findSourceDefinition",
         Status = "status",
         TypeDefinition = "typeDefinition",
         ProjectInfo = "projectInfo",
@@ -904,6 +905,10 @@ namespace ts.server.protocol {
         readonly command: CommandTypes.DefinitionAndBoundSpan;
     }
 
+    export interface FindSourceDefinitionRequest extends FileLocationRequest {
+        readonly command: CommandTypes.FindSourceDefinition;
+    }
+
     export interface DefinitionAndBoundSpanResponse extends Response {
         readonly body: DefinitionInfoAndBoundSpan;
     }
diff --git a/src/server/scriptInfo.ts b/src/server/scriptInfo.ts
index a26fcc6748932..dedd30fe19b93 100644
--- a/src/server/scriptInfo.ts
+++ b/src/server/scriptInfo.ts
@@ -501,7 +501,7 @@ namespace ts.server {
                 case 0:
                     return Errors.ThrowNoProject();
                 case 1:
-                    return ensureNotAutoImportProvider(this.containingProjects[0]);
+                    return ensurePrimaryProjectKind(this.containingProjects[0]);
                 default:
                     // If this file belongs to multiple projects, below is the order in which default project is used
                     // - for open script info, its default configured project during opening is default if info is part of it
@@ -536,7 +536,7 @@ namespace ts.server {
                             firstInferredProject = project;
                         }
                     }
-                    return ensureNotAutoImportProvider(defaultConfiguredProject ||
+                    return ensurePrimaryProjectKind(defaultConfiguredProject ||
                         firstNonSourceOfProjectReferenceRedirect ||
                         firstConfiguredProject ||
                         firstExternalProject ||
@@ -622,8 +622,10 @@ namespace ts.server {
         }
 
         /*@internal*/
-        isContainedByAutoImportProvider() {
-            return some(this.containingProjects, p => p.projectKind === ProjectKind.AutoImportProvider);
+        isContainedByBackgroundProject() {
+            return some(
+                this.containingProjects,
+                p => p.projectKind === ProjectKind.AutoImportProvider || p.projectKind === ProjectKind.Auxiliary);
         }
 
         /**
@@ -669,8 +671,13 @@ namespace ts.server {
         }
     }
 
-    function ensureNotAutoImportProvider(project: Project | undefined) {
-        if (!project || project.projectKind === ProjectKind.AutoImportProvider) {
+    /**
+     * Throws an error if `project` is an AutoImportProvider or AuxiliaryProject,
+     * which are used in the background by other Projects and should never be
+     * reported as the default project for a ScriptInfo.
+     */
+    function ensurePrimaryProjectKind(project: Project | undefined) {
+        if (!project || project.projectKind === ProjectKind.AutoImportProvider || project.projectKind === ProjectKind.Auxiliary) {
             return Errors.ThrowNoProject();
         }
         return project;
diff --git a/src/server/session.ts b/src/server/session.ts
index 16b04c6979220..3345f362eda77 100644
--- a/src/server/session.ts
+++ b/src/server/session.ts
@@ -1193,6 +1193,7 @@ namespace ts.server {
                     containerName: info.containerName,
                     kind: info.kind,
                     name: info.name,
+                    failedAliasResolution: info.failedAliasResolution,
                     ...info.unverified && { unverified: info.unverified },
                 };
             });
@@ -1228,6 +1229,159 @@ namespace ts.server {
             };
         }
 
+        private findSourceDefinition(args: protocol.FileLocationRequestArgs): readonly protocol.DefinitionInfo[] {
+            const { file, project } = this.getFileAndProject(args);
+            const position = this.getPositionInFile(args, file);
+            const unmappedDefinitions = project.getLanguageService().getDefinitionAtPosition(file, position);
+            let definitions: readonly DefinitionInfo[] = this.mapDefinitionInfoLocations(unmappedDefinitions || emptyArray, project).slice();
+            const needsJsResolution = this.projectService.serverMode === LanguageServiceMode.Semantic && (
+                !some(definitions, d => toNormalizedPath(d.fileName) !== file && !d.isAmbient) ||
+                some(definitions, d => !!d.failedAliasResolution));
+
+            if (needsJsResolution) {
+                const definitionSet = createSet<DefinitionInfo>(d => d.textSpan.start, documentSpansEqual);
+                definitions?.forEach(d => definitionSet.add(d));
+                const noDtsProject = project.getNoDtsResolutionProject([file]);
+                const ls = noDtsProject.getLanguageService();
+                const jsDefinitions = ls.getDefinitionAtPosition(file, position, /*searchOtherFilesOnly*/ true)
+                    ?.filter(d => toNormalizedPath(d.fileName) !== file);
+                if (some(jsDefinitions)) {
+                    for (const jsDefinition of jsDefinitions) {
+                        if (jsDefinition.unverified) {
+                            const refined = tryRefineDefinition(jsDefinition, project.getLanguageService().getProgram()!, ls.getProgram()!);
+                            if (some(refined)) {
+                                for (const def of refined) {
+                                    definitionSet.add(def);
+                                }
+                                continue;
+                            }
+                        }
+                        definitionSet.add(jsDefinition);
+                    }
+                }
+                else {
+                    const ambientCandidates = definitions.filter(d => toNormalizedPath(d.fileName) !== file && d.isAmbient);
+                    for (const candidate of some(ambientCandidates) ? ambientCandidates : getAmbientCandidatesByClimbingAccessChain()) {
+                        const fileNameToSearch = findImplementationFileFromDtsFileName(candidate.fileName, file, noDtsProject);
+                        if (!fileNameToSearch || !ensureRoot(noDtsProject, fileNameToSearch)) {
+                            continue;
+                        }
+                        const noDtsProgram = ls.getProgram()!;
+                        const fileToSearch = Debug.checkDefined(noDtsProgram.getSourceFile(fileNameToSearch));
+                        for (const match of searchForDeclaration(candidate.name, fileToSearch, noDtsProgram)) {
+                            definitionSet.add(match);
+                        }
+                    }
+                }
+                definitions = arrayFrom(definitionSet.values());
+            }
+
+            definitions = definitions.filter(d => !d.isAmbient && !d.failedAliasResolution);
+            return this.mapDefinitionInfo(definitions, project);
+
+            function findImplementationFileFromDtsFileName(fileName: string, resolveFromFile: string, auxiliaryProject: Project) {
+                const nodeModulesPathParts = getNodeModulePathParts(fileName);
+                if (nodeModulesPathParts && fileName.lastIndexOf(nodeModulesPathPart) === nodeModulesPathParts.topLevelNodeModulesIndex) {
+                    // Second check ensures the fileName only contains one `/node_modules/`. If there's more than one I give up.
+                    const packageDirectory = fileName.substring(0, nodeModulesPathParts.packageRootIndex);
+                    const packageJsonCache = project.getModuleResolutionCache()?.getPackageJsonInfoCache();
+                    const compilerOptions = project.getCompilationSettings();
+                    const packageJson = getPackageScopeForPath(project.toPath(packageDirectory + "/package.json"), packageJsonCache, project, compilerOptions);
+                    if (!packageJson) return undefined;
+                    // Use fake options instead of actual compiler options to avoid following export map if the project uses node12 or nodenext -
+                    // Mapping from an export map entry across packages is out of scope for now. Returned entrypoints will only be what can be
+                    // resolved from the package root under --moduleResolution node
+                    const entrypoints = getEntrypointsFromPackageJsonInfo(
+                        packageJson,
+                        { moduleResolution: ModuleResolutionKind.NodeJs },
+                        project,
+                        project.getModuleResolutionCache());
+                    // This substring is correct only because we checked for a single `/node_modules/` at the top.
+                    const packageNamePathPart = fileName.substring(
+                        nodeModulesPathParts.topLevelPackageNameIndex + 1,
+                        nodeModulesPathParts.packageRootIndex);
+                    const packageName = getPackageNameFromTypesPackageName(unmangleScopedPackageName(packageNamePathPart));
+                    const path = project.toPath(fileName);
+                    if (entrypoints && some(entrypoints, e => project.toPath(e) === path)) {
+                        // This file was the main entrypoint of a package. Try to resolve that same package name with
+                        // the auxiliary project that only resolves to implementation files.
+                        const [implementationResolution] = auxiliaryProject.resolveModuleNames([packageName], resolveFromFile);
+                        return implementationResolution?.resolvedFileName;
+                    }
+                    else {
+                        // It wasn't the main entrypoint but we are in node_modules. Try a subpath into the package.
+                        const pathToFileInPackage = fileName.substring(nodeModulesPathParts.packageRootIndex + 1);
+                        const specifier = `${packageName}/${removeFileExtension(pathToFileInPackage)}`;
+                        const [implementationResolution] = auxiliaryProject.resolveModuleNames([specifier], resolveFromFile);
+                        return implementationResolution?.resolvedFileName;
+                    }
+                }
+                // We're not in node_modules, and we only get to this function if non-dts module resolution failed.
+                // I'm not sure what else I can do here that isn't already covered by that module resolution.
+                return undefined;
+            }
+
+            // In 'foo.bar./**/baz', if we got not results on 'baz', see if we can get an ambient definition
+            // for 'bar' or 'foo' (in that order) so we can search for declarations of 'baz' later.
+            function getAmbientCandidatesByClimbingAccessChain(): readonly { name: string, fileName: string }[] {
+                const ls = project.getLanguageService();
+                const program = ls.getProgram()!;
+                const initialNode = getTouchingPropertyName(program.getSourceFile(file)!, position);
+                if ((isStringLiteralLike(initialNode) || isIdentifier(initialNode)) && isAccessExpression(initialNode.parent)) {
+                    return forEachNameInAccessChainWalkingLeft(initialNode, nameInChain => {
+                        if (nameInChain === initialNode) return undefined;
+                        const candidates = ls.getDefinitionAtPosition(file, nameInChain.getStart(), /*searchOtherFilesOnly*/ true)
+                            ?.filter(d => toNormalizedPath(d.fileName) !== file && d.isAmbient)
+                            .map(d => ({
+                                fileName: d.fileName,
+                                name: getTextOfIdentifierOrLiteral(initialNode)
+                            }));
+                        if (some(candidates)) {
+                            return candidates;
+                        }
+                    }) || emptyArray;
+                }
+                return emptyArray;
+            }
+
+            function tryRefineDefinition(definition: DefinitionInfo, program: Program, noDtsProgram: Program) {
+                const fileToSearch = noDtsProgram.getSourceFile(definition.fileName);
+                if (!fileToSearch) {
+                    return undefined;
+                }
+                const initialNode = getTouchingPropertyName(program.getSourceFile(file)!, position);
+                const symbol = program.getTypeChecker().getSymbolAtLocation(initialNode);
+                const importSpecifier = symbol && getDeclarationOfKind<ImportSpecifier>(symbol, SyntaxKind.ImportSpecifier);
+                if (!importSpecifier) return undefined;
+
+                const nameToSearch = importSpecifier.propertyName?.text || importSpecifier.name.text;
+                return searchForDeclaration(nameToSearch, fileToSearch, noDtsProgram);
+            }
+
+            function searchForDeclaration(declarationName: string, fileToSearch: SourceFile, noDtsProgram: Program) {
+                const matches = FindAllReferences.Core.getTopMostDeclarationNamesInFile(declarationName, fileToSearch);
+                return mapDefined(matches, match => {
+                    const symbol = noDtsProgram.getTypeChecker().getSymbolAtLocation(match);
+                    const decl = getDeclarationFromName(match);
+                    if (symbol && decl) {
+                        // I think the last argument to this is supposed to be the start node, but it doesn't seem important.
+                        // Callers internal to GoToDefinition already get confused about this.
+                        return GoToDefinition.createDefinitionInfo(decl, noDtsProgram.getTypeChecker(), symbol, decl, /*unverified*/ true);
+                    }
+                });
+            }
+
+            function ensureRoot(project: Project, fileName: string) {
+                const info = project.getScriptInfo(fileName);
+                if (!info) return false;
+                if (!project.containsScriptInfo(info)) {
+                    project.addRoot(info);
+                    project.updateGraph();
+                }
+                return true;
+            }
+        }
+
         private getEmitOutput(args: protocol.EmitOutputRequestArgs): EmitOutput | protocol.EmitOutput {
             const { file, project } = this.getFileAndProject(args);
             if (!project.shouldEmitFile(project.getScriptInfo(file))) {
@@ -2751,12 +2905,15 @@ namespace ts.server {
             [CommandNames.DefinitionFull]: (request: protocol.DefinitionRequest) => {
                 return this.requiredResponse(this.getDefinition(request.arguments, /*simplifiedResult*/ false));
             },
-            [CommandNames.DefinitionAndBoundSpan]: (request: protocol.DefinitionRequest) => {
+            [CommandNames.DefinitionAndBoundSpan]: (request: protocol.DefinitionAndBoundSpanRequest) => {
                 return this.requiredResponse(this.getDefinitionAndBoundSpan(request.arguments, /*simplifiedResult*/ true));
             },
-            [CommandNames.DefinitionAndBoundSpanFull]: (request: protocol.DefinitionRequest) => {
+            [CommandNames.DefinitionAndBoundSpanFull]: (request: protocol.DefinitionAndBoundSpanRequest) => {
                 return this.requiredResponse(this.getDefinitionAndBoundSpan(request.arguments, /*simplifiedResult*/ false));
             },
+            [CommandNames.FindSourceDefinition]: (request: protocol.FindSourceDefinitionRequest) => {
+                return this.requiredResponse(this.findSourceDefinition(request.arguments));
+            },
             [CommandNames.EmitOutput]: (request: protocol.EmitOutputRequest) => {
                 return this.requiredResponse(this.getEmitOutput(request.arguments));
             },
diff --git a/src/server/utilitiesPublic.ts b/src/server/utilitiesPublic.ts
index 026162133bcbb..7b28a9b17d367 100644
--- a/src/server/utilitiesPublic.ts
+++ b/src/server/utilitiesPublic.ts
@@ -122,6 +122,11 @@ namespace ts.server {
         return `/dev/null/autoImportProviderProject${counter}*`;
     }
 
+    /*@internal*/
+    export function makeAuxiliaryProjectName(counter: number): string {
+        return `/dev/null/auxiliaryProject${counter}*`;
+    }
+
     export function createSortedArray<T>(): SortedArray<T> {
         return [] as any as SortedArray<T>; // TODO: GH#19873
     }
diff --git a/src/services/codefixes/importFixes.ts b/src/services/codefixes/importFixes.ts
index 485fc02727990..f4e8b416f0590 100644
--- a/src/services/codefixes/importFixes.ts
+++ b/src/services/codefixes/importFixes.ts
@@ -453,7 +453,7 @@ namespace ts.codefix {
         // and it is up to the user to decide which one fits best.
         return firstDefined(existingImports, ({ declaration }): FixUseNamespaceImport | undefined => {
             const namespacePrefix = getNamespaceLikeImportText(declaration);
-            const moduleSpecifier = tryGetModuleSpecifierFromDeclaration(declaration);
+            const moduleSpecifier = tryGetModuleSpecifierFromDeclaration(declaration)?.text;
             if (namespacePrefix && moduleSpecifier) {
                 const moduleSymbol = getTargetModuleFromNamespaceLikeImport(declaration, checker);
                 if (moduleSymbol && moduleSymbol.exports!.has(escapeLeadingUnderscores(symbolName))) {
@@ -670,7 +670,7 @@ namespace ts.codefix {
         checker: TypeChecker,
         compilerOptions: CompilerOptions
     ): FixAddNewImport | undefined {
-        const moduleSpecifier = tryGetModuleSpecifierFromDeclaration(declaration);
+        const moduleSpecifier = tryGetModuleSpecifierFromDeclaration(declaration)?.text;
         if (moduleSpecifier) {
             const addAsTypeOnly = useRequire
                 ? AddAsTypeOnly.NotAllowed
diff --git a/src/services/findAllReferences.ts b/src/services/findAllReferences.ts
index aeac984d5dbb7..ba46b5d311f46 100644
--- a/src/services/findAllReferences.ts
+++ b/src/services/findAllReferences.ts
@@ -1365,6 +1365,31 @@ namespace ts.FindAllReferences {
             }
         }
 
+        export function getTopMostDeclarationNamesInFile(declarationName: string, sourceFile: SourceFile): readonly Node[] {
+            const candidates = filter(getPossibleSymbolReferenceNodes(sourceFile, declarationName), name => !!getDeclarationFromName(name));
+            return candidates.reduce((topMost, decl) => {
+                const depth = getDepth(decl);
+                if (!some(topMost.declarationNames) || depth === topMost.depth) {
+                    topMost.declarationNames.push(decl);
+                    topMost.depth = depth;
+                }
+                else if (depth < topMost.depth) {
+                    topMost.declarationNames = [decl];
+                    topMost.depth = depth;
+                }
+                return topMost;
+            }, { depth: Infinity, declarationNames: [] as Node[] }).declarationNames;
+
+            function getDepth(declaration: Node | undefined) {
+                let depth = 0;
+                while (declaration) {
+                    declaration = getContainerNode(declaration);
+                    depth++;
+                }
+                return depth;
+            }
+        }
+
         export function someSignatureUsage(
             signature: SignatureDeclaration,
             sourceFiles: readonly SourceFile[],
diff --git a/src/services/goToDefinition.ts b/src/services/goToDefinition.ts
index 3b49ec4b64610..b1c8ec28c1b8d 100644
--- a/src/services/goToDefinition.ts
+++ b/src/services/goToDefinition.ts
@@ -1,6 +1,6 @@
 /* @internal */
 namespace ts.GoToDefinition {
-    export function getDefinitionAtPosition(program: Program, sourceFile: SourceFile, position: number): readonly DefinitionInfo[] | undefined {
+    export function getDefinitionAtPosition(program: Program, sourceFile: SourceFile, position: number, searchOtherFilesOnly?: boolean): readonly DefinitionInfo[] | undefined {
         const resolvedRef = getReferenceAtPosition(sourceFile, position, program);
         const fileReferenceDefinition = resolvedRef && [getDefinitionInfoForFileReference(resolvedRef.reference.fileName, resolvedRef.fileName, resolvedRef.unverified)] || emptyArray;
         if (resolvedRef?.file) {
@@ -28,18 +28,49 @@ namespace ts.GoToDefinition {
 
         if (isStaticModifier(node) && isClassStaticBlockDeclaration(node.parent)) {
             const classDecl = node.parent.parent;
-            const symbol = getSymbol(classDecl, typeChecker);
+            const { symbol, failedAliasResolution } = getSymbol(classDecl, typeChecker);
+
             const staticBlocks = filter(classDecl.members, isClassStaticBlockDeclaration);
             const containerName = symbol ? typeChecker.symbolToString(symbol, classDecl) : "";
             const sourceFile = node.getSourceFile();
             return map(staticBlocks, staticBlock => {
                 let { pos } = moveRangePastModifiers(staticBlock);
                 pos = skipTrivia(sourceFile.text, pos);
-                return createDefinitionInfoFromName(typeChecker, staticBlock, ScriptElementKind.constructorImplementationElement, "static {}", containerName, { start: pos, length: "static".length });
+                return createDefinitionInfoFromName(typeChecker, staticBlock, ScriptElementKind.constructorImplementationElement, "static {}", containerName, /*unverified*/ false, failedAliasResolution, { start: pos, length: "static".length });
             });
         }
 
-        const symbol = getSymbol(node, typeChecker);
+        let { symbol, failedAliasResolution } = getSymbol(node, typeChecker);
+        let fallbackNode = node;
+
+        if (searchOtherFilesOnly && failedAliasResolution) {
+            // We couldn't resolve the specific import, try on the module specifier.
+            const importDeclaration = forEach([node, ...symbol?.declarations || emptyArray], n => findAncestor(n, isAnyImportOrBareOrAccessedRequire));
+            const moduleSpecifier = importDeclaration && tryGetModuleSpecifierFromDeclaration(importDeclaration);
+            if (moduleSpecifier) {
+                ({ symbol, failedAliasResolution } = getSymbol(moduleSpecifier, typeChecker));
+                fallbackNode = moduleSpecifier;
+            }
+        }
+
+        if (!symbol && isModuleSpecifierLike(fallbackNode)) {
+            // We couldn't resolve the module specifier as an external module, but it could
+            // be that module resolution succeeded but the target was not a module.
+            const ref = sourceFile.resolvedModules?.get(fallbackNode.text, getModeForUsageLocation(sourceFile, fallbackNode));
+            if (ref) {
+                return [{
+                    name: fallbackNode.text,
+                    fileName: ref.resolvedFileName,
+                    containerName: undefined!,
+                    containerKind: undefined!,
+                    kind: ScriptElementKind.scriptElement,
+                    textSpan: createTextSpan(0, 0),
+                    failedAliasResolution,
+                    isAmbient: isDeclarationFileName(ref.resolvedFileName),
+                    unverified: fallbackNode !== node,
+                }];
+            }
+        }
 
         // Could not find a symbol e.g. node is string or number keyword,
         // or the symbol was an internal symbol and does not have a declaration e.g. undefined symbol
@@ -47,17 +78,19 @@ namespace ts.GoToDefinition {
             return concatenate(fileReferenceDefinition, getDefinitionInfoForIndexSignatures(node, typeChecker));
         }
 
+        if (searchOtherFilesOnly && every(symbol.declarations, d => d.getSourceFile().fileName === sourceFile.fileName)) return undefined;
+
         const calledDeclaration = tryGetSignatureDeclaration(typeChecker, node);
         // Don't go to the component constructor definition for a JSX element, just go to the component definition.
         if (calledDeclaration && !(isJsxOpeningLikeElement(node.parent) && isConstructorLike(calledDeclaration))) {
-            const sigInfo = createDefinitionFromSignatureDeclaration(typeChecker, calledDeclaration);
+            const sigInfo = createDefinitionFromSignatureDeclaration(typeChecker, calledDeclaration, failedAliasResolution);
             // For a function, if this is the original function definition, return just sigInfo.
             // If this is the original constructor definition, parent is the class.
             if (typeChecker.getRootSymbols(symbol).some(s => symbolMatchesSignature(s, calledDeclaration))) {
                 return [sigInfo];
             }
             else {
-                const defs = getDefinitionFromSymbol(typeChecker, symbol, node, calledDeclaration) || emptyArray;
+                const defs = getDefinitionFromSymbol(typeChecker, symbol, node, failedAliasResolution, calledDeclaration) || emptyArray;
                 // For a 'super()' call, put the signature first, else put the variable first.
                 return node.kind === SyntaxKind.SuperKeyword ? [sigInfo, ...defs] : [...defs, sigInfo];
             }
@@ -70,7 +103,7 @@ namespace ts.GoToDefinition {
         // assignment. This case and others are handled by the following code.
         if (node.parent.kind === SyntaxKind.ShorthandPropertyAssignment) {
             const shorthandSymbol = typeChecker.getShorthandAssignmentValueSymbol(symbol.valueDeclaration);
-            const definitions = shorthandSymbol?.declarations ? shorthandSymbol.declarations.map(decl => createDefinitionInfo(decl, typeChecker, shorthandSymbol, node)) : emptyArray;
+            const definitions = shorthandSymbol?.declarations ? shorthandSymbol.declarations.map(decl => createDefinitionInfo(decl, typeChecker, shorthandSymbol, node, /*unverified*/ false, failedAliasResolution)) : emptyArray;
             return concatenate(definitions, getDefinitionFromObjectLiteralElement(typeChecker, node) || emptyArray);
         }
 
@@ -95,7 +128,7 @@ namespace ts.GoToDefinition {
             });
         }
 
-        return concatenate(fileReferenceDefinition, getDefinitionFromObjectLiteralElement(typeChecker, node) || getDefinitionFromSymbol(typeChecker, symbol, node));
+        return concatenate(fileReferenceDefinition, getDefinitionFromObjectLiteralElement(typeChecker, node) || getDefinitionFromSymbol(typeChecker, symbol, node, failedAliasResolution));
     }
 
     /**
@@ -199,25 +232,25 @@ namespace ts.GoToDefinition {
         }
 
         if (isImportMeta(node.parent) && node.parent.name === node) {
-            return definitionFromType(typeChecker.getTypeAtLocation(node.parent), typeChecker, node.parent);
+            return definitionFromType(typeChecker.getTypeAtLocation(node.parent), typeChecker, node.parent, /*failedAliasResolution*/ false);
         }
 
-        const symbol = getSymbol(node, typeChecker);
+        const { symbol, failedAliasResolution } = getSymbol(node, typeChecker);
         if (!symbol) return undefined;
 
         const typeAtLocation = typeChecker.getTypeOfSymbolAtLocation(symbol, node);
         const returnType = tryGetReturnTypeOfFunction(symbol, typeAtLocation, typeChecker);
-        const fromReturnType = returnType && definitionFromType(returnType, typeChecker, node);
+        const fromReturnType = returnType && definitionFromType(returnType, typeChecker, node, failedAliasResolution);
         // If a function returns 'void' or some other type with no definition, just return the function definition.
-        const typeDefinitions = fromReturnType && fromReturnType.length !== 0 ? fromReturnType : definitionFromType(typeAtLocation, typeChecker, node);
+        const typeDefinitions = fromReturnType && fromReturnType.length !== 0 ? fromReturnType : definitionFromType(typeAtLocation, typeChecker, node, failedAliasResolution);
         return typeDefinitions.length ? typeDefinitions
-            : !(symbol.flags & SymbolFlags.Value) && symbol.flags & SymbolFlags.Type ? getDefinitionFromSymbol(typeChecker, skipAlias(symbol, typeChecker), node)
+            : !(symbol.flags & SymbolFlags.Value) && symbol.flags & SymbolFlags.Type ? getDefinitionFromSymbol(typeChecker, skipAlias(symbol, typeChecker), node, failedAliasResolution)
             : undefined;
     }
 
-    function definitionFromType(type: Type, checker: TypeChecker, node: Node): readonly DefinitionInfo[] {
+    function definitionFromType(type: Type, checker: TypeChecker, node: Node, failedAliasResolution: boolean | undefined): readonly DefinitionInfo[] {
         return flatMap(type.isUnion() && !(type.flags & TypeFlags.Enum) ? type.types : [type], t =>
-            t.symbol && getDefinitionFromSymbol(checker, t.symbol, node));
+            t.symbol && getDefinitionFromSymbol(checker, t.symbol, node, failedAliasResolution));
     }
 
     function tryGetReturnTypeOfFunction(symbol: Symbol, type: Type, checker: TypeChecker): Type | undefined {
@@ -259,19 +292,23 @@ namespace ts.GoToDefinition {
         return mapDefined(checker.getIndexInfosAtLocation(node), info => info.declaration && createDefinitionFromSignatureDeclaration(checker, info.declaration));
     }
 
-    function getSymbol(node: Node, checker: TypeChecker): Symbol | undefined {
+    function getSymbol(node: Node, checker: TypeChecker) {
         const symbol = checker.getSymbolAtLocation(node);
         // If this is an alias, and the request came at the declaration location
         // get the aliased symbol instead. This allows for goto def on an import e.g.
         //   import {A, B} from "mod";
         // to jump to the implementation directly.
+        let failedAliasResolution = false;
         if (symbol?.declarations && symbol.flags & SymbolFlags.Alias && shouldSkipAlias(node, symbol.declarations[0])) {
             const aliased = checker.getAliasedSymbol(symbol);
             if (aliased.declarations) {
-                return aliased;
+                return { symbol: aliased };
+            }
+            else {
+                failedAliasResolution = true;
             }
         }
-        return symbol;
+        return { symbol, failedAliasResolution };
     }
 
     // Go to the original declaration for cases:
@@ -286,28 +323,44 @@ namespace ts.GoToDefinition {
         if (node.parent === declaration) {
             return true;
         }
-        switch (declaration.kind) {
-            case SyntaxKind.ImportClause:
-            case SyntaxKind.ImportEqualsDeclaration:
-                return true;
-            case SyntaxKind.ImportSpecifier:
-                return declaration.parent.kind === SyntaxKind.NamedImports;
-            case SyntaxKind.BindingElement:
-            case SyntaxKind.VariableDeclaration:
-                return isInJSFile(declaration) && isVariableDeclarationInitializedToBareOrAccessedRequire(declaration);
-            default:
-                return false;
+        if (declaration.kind === SyntaxKind.NamespaceImport) {
+            return false;
         }
+        return true;
+    }
+
+    /**
+     * ```ts
+     * function f() {}
+     * f.foo = 0;
+     * ```
+     *
+     * Here, `f` has two declarations: the function declaration, and the identifier in the next line.
+     * The latter is a declaration for `f` because it gives `f` the `SymbolFlags.Namespace` meaning so
+     * it can contain `foo`. However, that declaration is pretty uninteresting and not intuitively a
+     * "definition" for `f`. Ideally, the question we'd like to answer is "what SymbolFlags does this
+     * declaration contribute to the symbol for `f`?" If the answer is just `Namespace` and the
+     * declaration looks like an assignment, that declaration is in no sense a definition for `f`.
+     * But that information is totally lost during binding and/or symbol merging, so we need to do
+     * our best to reconstruct it or use other heuristics. This function (and the logic around its
+     * calling) covers our tests but feels like a hack, and it would be great if someone could come
+     * up with a more precise definition of what counts as a definition.
+     */
+    function isExpandoDeclaration(node: Declaration): boolean {
+        if (!isAssignmentDeclaration(node)) return false;
+        const containingAssignment = findAncestor(node, p => {
+            if (isAssignmentExpression(p)) return true;
+            if (!isAssignmentDeclaration(p as Declaration)) return "quit";
+            return false;
+        }) as AssignmentExpression<AssignmentOperatorToken> | undefined;
+        return !!containingAssignment && getAssignmentDeclarationKind(containingAssignment) === AssignmentDeclarationKind.Property;
     }
 
-    function getDefinitionFromSymbol(typeChecker: TypeChecker, symbol: Symbol, node: Node, declarationNode?: Node): DefinitionInfo[] | undefined {
-        // There are cases when you extend a function by adding properties to it afterwards,
-        // we want to strip those extra properties.
-        // For deduping purposes, we also want to exclude any declarationNodes if provided.
-        const filteredDeclarations =
-            filter(symbol.declarations, d => d !== declarationNode && (!isAssignmentDeclaration(d) || d === symbol.valueDeclaration))
-            || undefined;
-        return getConstructSignatureDefinition() || getCallSignatureDefinition() || map(filteredDeclarations, declaration => createDefinitionInfo(declaration, typeChecker, symbol, node));
+    function getDefinitionFromSymbol(typeChecker: TypeChecker, symbol: Symbol, node: Node, failedAliasResolution?: boolean, excludeDeclaration?: Node): DefinitionInfo[] | undefined {
+        const filteredDeclarations = filter(symbol.declarations, d => d !== excludeDeclaration);
+        const withoutExpandos = filter(filteredDeclarations, d => !isExpandoDeclaration(d));
+        const results = some(withoutExpandos) ? withoutExpandos : filteredDeclarations;
+        return getConstructSignatureDefinition() || getCallSignatureDefinition() || map(results, declaration => createDefinitionInfo(declaration, typeChecker, symbol, node, /*unverified*/ false, failedAliasResolution));
 
         function getConstructSignatureDefinition(): DefinitionInfo[] | undefined {
             // Applicable only if we are in a new expression, or we are on a constructor declaration
@@ -335,21 +388,21 @@ namespace ts.GoToDefinition {
             return declarations.length
                 ? declarationsWithBody.length !== 0
                     ? declarationsWithBody.map(x => createDefinitionInfo(x, typeChecker, symbol, node))
-                    : [createDefinitionInfo(last(declarations), typeChecker, symbol, node)]
+                    : [createDefinitionInfo(last(declarations), typeChecker, symbol, node, /*unverified*/ false, failedAliasResolution)]
                 : undefined;
         }
     }
 
     /** Creates a DefinitionInfo from a Declaration, using the declaration's name if possible. */
-    function createDefinitionInfo(declaration: Declaration, checker: TypeChecker, symbol: Symbol, node: Node): DefinitionInfo {
+    export function createDefinitionInfo(declaration: Declaration, checker: TypeChecker, symbol: Symbol, node: Node, unverified?: boolean, failedAliasResolution?: boolean): DefinitionInfo {
         const symbolName = checker.symbolToString(symbol); // Do not get scoped name, just the name of the symbol
         const symbolKind = SymbolDisplay.getSymbolKind(checker, symbol, node);
         const containerName = symbol.parent ? checker.symbolToString(symbol.parent, node) : "";
-        return createDefinitionInfoFromName(checker, declaration, symbolKind, symbolName, containerName);
+        return createDefinitionInfoFromName(checker, declaration, symbolKind, symbolName, containerName, unverified, failedAliasResolution);
     }
 
     /** Creates a DefinitionInfo directly from the name of a declaration. */
-    function createDefinitionInfoFromName(checker: TypeChecker, declaration: Declaration, symbolKind: ScriptElementKind, symbolName: string, containerName: string, textSpan?: TextSpan): DefinitionInfo {
+    function createDefinitionInfoFromName(checker: TypeChecker, declaration: Declaration, symbolKind: ScriptElementKind, symbolName: string, containerName: string, unverified?: boolean, failedAliasResolution?: boolean, textSpan?: TextSpan): DefinitionInfo {
         const sourceFile = declaration.getSourceFile();
         if (!textSpan) {
             const name = getNameOfDeclaration(declaration) || declaration;
@@ -367,7 +420,10 @@ namespace ts.GoToDefinition {
                 sourceFile,
                 FindAllReferences.getContextNode(declaration)
             ),
-            isLocal: !isDefinitionVisible(checker, declaration)
+            isLocal: !isDefinitionVisible(checker, declaration),
+            isAmbient: !!(declaration.flags & NodeFlags.Ambient),
+            unverified,
+            failedAliasResolution,
         };
     }
 
@@ -402,8 +458,8 @@ namespace ts.GoToDefinition {
         }
     }
 
-    function createDefinitionFromSignatureDeclaration(typeChecker: TypeChecker, decl: SignatureDeclaration): DefinitionInfo {
-        return createDefinitionInfo(decl, typeChecker, decl.symbol, decl);
+    function createDefinitionFromSignatureDeclaration(typeChecker: TypeChecker, decl: SignatureDeclaration, failedAliasResolution?: boolean): DefinitionInfo {
+        return createDefinitionInfo(decl, typeChecker, decl.symbol, decl, /*unverified*/ false, failedAliasResolution);
     }
 
     export function findReferenceInPosition(refs: readonly FileReference[], pos: number): FileReference | undefined {
diff --git a/src/services/services.ts b/src/services/services.ts
index 9e39d4a405330..20373fa773e91 100644
--- a/src/services/services.ts
+++ b/src/services/services.ts
@@ -1768,9 +1768,9 @@ namespace ts {
         }
 
         /// Goto definition
-        function getDefinitionAtPosition(fileName: string, position: number): readonly DefinitionInfo[] | undefined {
+        function getDefinitionAtPosition(fileName: string, position: number, searchOtherFilesOnly?: boolean): readonly DefinitionInfo[] | undefined {
             synchronizeHostData();
-            return GoToDefinition.getDefinitionAtPosition(program, getValidSourceFile(fileName), position);
+            return GoToDefinition.getDefinitionAtPosition(program, getValidSourceFile(fileName), position, searchOtherFilesOnly);
         }
 
         function getDefinitionAndBoundSpan(fileName: string, position: number): DefinitionInfoAndBoundSpan | undefined {
diff --git a/src/services/types.ts b/src/services/types.ts
index 54bd93f7f32ce..f84ee7d3ca16b 100644
--- a/src/services/types.ts
+++ b/src/services/types.ts
@@ -472,6 +472,9 @@ namespace ts {
 
         getSmartSelectionRange(fileName: string, position: number): SelectionRange;
 
+        /*@internal*/
+        // eslint-disable-next-line @typescript-eslint/unified-signatures
+        getDefinitionAtPosition(fileName: string, position: number, searchOtherFilesOnly: boolean): readonly DefinitionInfo[] | undefined;
         getDefinitionAtPosition(fileName: string, position: number): readonly DefinitionInfo[] | undefined;
         getDefinitionAndBoundSpan(fileName: string, position: number): DefinitionInfoAndBoundSpan | undefined;
         getTypeDefinitionAtPosition(fileName: string, position: number): readonly DefinitionInfo[] | undefined;
@@ -1032,6 +1035,8 @@ namespace ts {
         containerName: string;
         unverified?: boolean;
         /* @internal */ isLocal?: boolean;
+        /* @internal */ isAmbient?: boolean;
+        /* @internal */ failedAliasResolution?: boolean;
     }
 
     export interface DefinitionInfoAndBoundSpan {
diff --git a/src/testRunner/tsconfig.json b/src/testRunner/tsconfig.json
index 2caa9873f29dd..599aaa19a8615 100644
--- a/src/testRunner/tsconfig.json
+++ b/src/testRunner/tsconfig.json
@@ -169,6 +169,7 @@
         "unittests/tscWatch/watchEnvironment.ts",
         "unittests/tsserver/applyChangesToOpenFiles.ts",
         "unittests/tsserver/autoImportProvider.ts",
+        "unittests/tsserver/auxiliaryProject.ts",
         "unittests/tsserver/cachingFileSystemInformation.ts",
         "unittests/tsserver/cancellationToken.ts",
         "unittests/tsserver/compileOnSave.ts",
diff --git a/src/testRunner/unittests/tsserver/auxiliaryProject.ts b/src/testRunner/unittests/tsserver/auxiliaryProject.ts
new file mode 100644
index 0000000000000..4b5f865c19aeb
--- /dev/null
+++ b/src/testRunner/unittests/tsserver/auxiliaryProject.ts
@@ -0,0 +1,47 @@
+namespace ts.projectSystem {
+    const aTs: File = {
+        path: "/a.ts",
+        content: `import { B } from "./b";`
+    };
+    const bDts: File = {
+        path: "/b.d.ts",
+        content: `export declare class B {}`
+    };
+    const bJs: File = {
+        path: "/b.js",
+        content: `export class B {}`
+    };
+    describe("unittests:: tsserver:: auxiliaryProject", () => {
+        it("AuxiliaryProject does not remove scrips from InferredProject", () => {
+            const host = createServerHost([aTs, bDts, bJs]);
+            const session = createSession(host);
+            const projectService = session.getProjectService();
+            openFilesForSession([aTs], session);
+
+            // Open file is in inferred project
+            checkNumberOfInferredProjects(projectService, 1);
+            const inferredProject = projectService.inferredProjects[0];
+
+            // getNoDtsResolutionProject will create an AuxiliaryProject with a.ts and b.js
+            const auxProject = inferredProject.getNoDtsResolutionProject([aTs.path]);
+            auxProject.updateGraph();
+
+            // b.js ScriptInfo is now available because it's contained by the AuxiliaryProject.
+            // The AuxiliaryProject should never be the default project for anything, so
+            // the ScriptInfo should still report being an orphan, and getting its default
+            // project should throw.
+            const bJsScriptInfo = Debug.checkDefined(projectService.getScriptInfo(bJs.path));
+            assert(bJsScriptInfo.isOrphan());
+            assert(bJsScriptInfo.isContainedByBackgroundProject());
+            assert.deepEqual(bJsScriptInfo.containingProjects, [auxProject]);
+            assert.throws(() => bJsScriptInfo.getDefaultProject());
+
+            // When b.js is opened in the editor, it should be put into an InferredProject
+            // even though it's still contained by the AuxiliaryProject.
+            openFilesForSession([bJs], session);
+            assert(!bJsScriptInfo.isOrphan());
+            assert(bJsScriptInfo.isContainedByBackgroundProject());
+            assert.equal(bJsScriptInfo.getDefaultProject().projectKind, server.ProjectKind.Inferred);
+        });
+    });
+}
diff --git a/tests/baselines/reference/api/tsserverlibrary.d.ts b/tests/baselines/reference/api/tsserverlibrary.d.ts
index 6146af8bb9f75..a08dea9d05f4a 100644
--- a/tests/baselines/reference/api/tsserverlibrary.d.ts
+++ b/tests/baselines/reference/api/tsserverlibrary.d.ts
@@ -7038,6 +7038,7 @@ declare namespace ts.server.protocol {
         Rename = "rename",
         Saveto = "saveto",
         SignatureHelp = "signatureHelp",
+        FindSourceDefinition = "findSourceDefinition",
         Status = "status",
         TypeDefinition = "typeDefinition",
         ProjectInfo = "projectInfo",
@@ -7657,6 +7658,9 @@ declare namespace ts.server.protocol {
     interface DefinitionAndBoundSpanRequest extends FileLocationRequest {
         readonly command: CommandTypes.DefinitionAndBoundSpan;
     }
+    interface FindSourceDefinitionRequest extends FileLocationRequest {
+        readonly command: CommandTypes.FindSourceDefinition;
+    }
     interface DefinitionAndBoundSpanResponse extends Response {
         readonly body: DefinitionInfoAndBoundSpan;
     }
@@ -9905,7 +9909,8 @@ declare namespace ts.server {
         Inferred = 0,
         Configured = 1,
         External = 2,
-        AutoImportProvider = 3
+        AutoImportProvider = 3,
+        Auxiliary = 4
     }
     function allRootFilesAreJsOrDts(project: Project): boolean;
     function allFilesAreJsOrDts(project: Project): boolean;
@@ -10640,6 +10645,7 @@ declare namespace ts.server {
         private getDefinition;
         private mapDefinitionInfoLocations;
         private getDefinitionAndBoundSpan;
+        private findSourceDefinition;
         private getEmitOutput;
         private mapJSDocTagInfo;
         private mapDisplayParts;
diff --git a/tests/baselines/reference/showConfig/Shows tsconfig for single option/noDtsResolution/tsconfig.json b/tests/baselines/reference/showConfig/Shows tsconfig for single option/noDtsResolution/tsconfig.json
new file mode 100644
index 0000000000000..a879d26a750ec
--- /dev/null
+++ b/tests/baselines/reference/showConfig/Shows tsconfig for single option/noDtsResolution/tsconfig.json	
@@ -0,0 +1,5 @@
+{
+    "compilerOptions": {
+        "noDtsResolution": true
+    }
+}
diff --git a/tests/cases/fourslash/fourslash.ts b/tests/cases/fourslash/fourslash.ts
index 486d24d4922a3..93aca2cfb1f89 100644
--- a/tests/cases/fourslash/fourslash.ts
+++ b/tests/cases/fourslash/fourslash.ts
@@ -317,6 +317,9 @@ declare namespace FourSlashInterface {
         goToDefinition(startsAndEnds: { [startMarkerName: string]: ArrayOrSingle<string> }): void;
         /** Verifies goToDefinition for each `${markerName}Reference` -> `${markerName}Definition` */
         goToDefinitionForMarkers(...markerNames: string[]): void;
+        goToSourceDefinition(startMarkerNames: ArrayOrSingle<string>, fileResult: { file: string, unverified?: boolean }): void;
+        goToSourceDefinition(startMarkerNames: ArrayOrSingle<string>, endMarkerNames: ArrayOrSingle<string | { marker: string, unverified?: boolean }>): void;
+        goToSourceDefinition(startMarkerNames: ArrayOrSingle<string>, endMarkerNames: ArrayOrSingle<string>): void;
         goToType(startsAndEnds: { [startMarkerName: string]: ArrayOrSingle<string> }): void;
         goToType(startMarkerNames: ArrayOrSingle<string>, endMarkerNames: ArrayOrSingle<string>): void;
         verifyGetEmitOutputForCurrentFile(expected: string): void;
diff --git a/tests/cases/fourslash/goToDefinitionExpandoElementAccess.ts b/tests/cases/fourslash/goToDefinitionExpandoElementAccess.ts
index 5deeb4c02532e..67efdb195e690 100644
--- a/tests/cases/fourslash/goToDefinitionExpandoElementAccess.ts
+++ b/tests/cases/fourslash/goToDefinitionExpandoElementAccess.ts
@@ -4,4 +4,4 @@
 ////f[/*0*/"x"] = 0;
 ////f[[|/*1*/"x"|]] = 1;
 
-verify.goToDefinition("1", "0");
+verify.goToDefinition("1", ["0", "1"]);
diff --git a/tests/cases/fourslash/quickInfoUntypedModuleImport.ts b/tests/cases/fourslash/quickInfoUntypedModuleImport.ts
index a25be52954766..15bc5000d1eff 100644
--- a/tests/cases/fourslash/quickInfoUntypedModuleImport.ts
+++ b/tests/cases/fourslash/quickInfoUntypedModuleImport.ts
@@ -1,7 +1,7 @@
 /// <reference path='fourslash.ts' />
 
 // @Filename: node_modules/foo/index.js
-////{}
+//// /*index*/{}
 
 // @Filename: a.ts
 ////import /*foo*/foo from /*fooModule*/"foo";
@@ -11,7 +11,7 @@ goTo.file("a.ts");
 verify.numberOfErrorsInCurrentFile(0);
 
 goTo.marker("fooModule");
-verify.goToDefinitionIs([]);
+verify.goToDefinitionIs(["index"]);
 verify.quickInfoIs("");
 
 goTo.marker("foo");
diff --git a/tests/cases/fourslash/server/goToSource10_mapFromAtTypes3.ts b/tests/cases/fourslash/server/goToSource10_mapFromAtTypes3.ts
new file mode 100644
index 0000000000000..6b636839e1781
--- /dev/null
+++ b/tests/cases/fourslash/server/goToSource10_mapFromAtTypes3.ts
@@ -0,0 +1,72 @@
+/// <reference path="../fourslash.ts" />
+
+// @moduleResolution: node
+
+// @Filename: /node_modules/lodash/package.json
+//// { "name": "lodash", "version": "4.17.15", "main": "./lodash.js" }
+
+// @Filename: /node_modules/lodash/lodash.js
+//// ;(function() {
+////     /**
+////      * Adds two numbers.
+////      *
+////      * @static
+////      * @memberOf _
+////      * @since 3.4.0
+////      * @category Math
+////      * @param {number} augend The first number in an addition.
+////      * @param {number} addend The second number in an addition.
+////      * @returns {number} Returns the total.
+////      * @example
+////      *
+////      * _.add(6, 4);
+////      * // => 10
+////      */
+////     var [|/*variable*/add|] = createMathOperation(function(augend, addend) {
+////      return augend + addend;
+////     }, 0);
+////
+////     function lodash(value) {}
+////     lodash.[|/*property*/add|] = add;
+////
+////     /** Detect free variable `global` from Node.js. */
+////     var freeGlobal = typeof global == 'object' && global && global.Object === Object && global;
+////     /** Detect free variable `self`. */
+////     var freeSelf = typeof self == 'object' && self && self.Object === Object && self;
+////     /** Used as a reference to the global object. */
+////     var root = freeGlobal || freeSelf || Function('return this')();
+////     /** Detect free variable `exports`. */
+////     var freeExports = typeof exports == 'object' && exports && !exports.nodeType && exports;////     
+////     /** Detect free variable `module`. */
+////     var freeModule = freeExports && typeof module == 'object' && module && !module.nodeType && module;
+////     if (freeModule) {
+////       // Export for Node.js.
+////       (freeModule.exports = _)._ = _;
+////       // Export for CommonJS support.
+////       freeExports._ = _;
+////     }
+////     else {
+////       // Export to the global object.
+////       root._ = _;
+////     }
+//// }.call(this));
+
+// @Filename: /node_modules/@types/lodash/package.json
+//// { "name": "@types/lodash", "version": "4.14.97", "types": "index.d.ts" }
+
+// @Filename: /node_modules/@types/lodash/index.d.ts
+//// export = _;
+//// export as namespace _;
+//// declare const _: _.LoDashStatic;
+//// declare namespace _ {
+////     interface LoDashStatic {}
+//// }
+
+
+// @Filename: /index.ts
+//// import { [|/*start*/add|] } from 'lodash';
+
+verify.goToSourceDefinition("start", [
+  { marker: "variable", unverified: true },
+  { marker: "property", unverified: true },
+]);
diff --git a/tests/cases/fourslash/server/goToSource11_propertyOfAlias.ts b/tests/cases/fourslash/server/goToSource11_propertyOfAlias.ts
new file mode 100644
index 0000000000000..9856d82610946
--- /dev/null
+++ b/tests/cases/fourslash/server/goToSource11_propertyOfAlias.ts
@@ -0,0 +1,15 @@
+/// <reference path="../fourslash.ts" />
+
+// @moduleResolution: node
+
+// @Filename: /a.js
+//// export const a = { /*end*/a: 'a' };
+
+// @Filename: /a.d.ts
+//// export declare const a: { a: string };
+
+// @Filename: /b.ts
+//// import { a } from './a';
+//// a.[|a/*start*/|]
+
+verify.goToSourceDefinition("start", "end");
diff --git a/tests/cases/fourslash/server/goToSource12_callbackParam.ts b/tests/cases/fourslash/server/goToSource12_callbackParam.ts
new file mode 100644
index 0000000000000..948591ba48e5f
--- /dev/null
+++ b/tests/cases/fourslash/server/goToSource12_callbackParam.ts
@@ -0,0 +1,43 @@
+/// <reference path="../fourslash.ts" />
+
+// @moduleResolution: node
+
+// Actual yargs doesn't work like this because the implementation package's
+// main entry exports a small function wrapper function whose return value
+// is derived from something imported from another file where all the
+// important code lives, and it's not quite clear on what grounds we can
+// make the jump from the main entry to that other file. But this test
+// demonstrates the need to do some filename-based jumping: regular go-to-def
+// on `yargs` goes to the type definition `Yargs`, but a JS-only go-to-def
+// simply stops on the callback parameter. So we have to start at the type
+// definition and say "well, maybe I can find a JS counterpart to this .d.ts
+// and just look for declarations called 'positional' in there."
+
+// @Filename: /node_modules/@types/yargs/package.json
+//// {
+////     "name": "@types/yargs",
+////     "version": "1.0.0",
+////     "types": "./index.d.ts"
+//// }
+
+// @Filename: /node_modules/@types/yargs/index.d.ts
+//// export interface Yargs { positional(): Yargs; }
+//// export declare function command(command: string, cb: (yargs: Yargs) => void): void;
+
+// @Filename: /node_modules/yargs/package.json
+//// {
+////     "name": "yargs",
+////     "version": "1.0.0",
+////     "main": "index.js"
+//// }
+
+// @Filename: /node_modules/yargs/index.js
+//// export function command(cmd, cb) { cb({ /*end*/positional: "This is obviously not even close to realistic" }); }
+
+// @Filename: /index.ts
+//// import { command } from "yargs";
+//// command("foo", yargs => {
+////     yargs.[|/*start*/positional|]();
+//// });
+
+verify.goToSourceDefinition("start", { marker: "end", unverified: true });
diff --git a/tests/cases/fourslash/server/goToSource1_localJsBesideDts.ts b/tests/cases/fourslash/server/goToSource1_localJsBesideDts.ts
new file mode 100644
index 0000000000000..5685db4076977
--- /dev/null
+++ b/tests/cases/fourslash/server/goToSource1_localJsBesideDts.ts
@@ -0,0 +1,14 @@
+/// <reference path="../fourslash.ts" />
+
+// @Filename: /a.js
+//// export const /*end*/a = "a";
+
+// @Filename: /a.d.ts
+//// export declare const a: string;
+
+// @Filename: /index.ts
+//// import { a } from [|"./a"/*moduleSpecifier*/|];
+//// [|a/*identifier*/|]
+
+verify.goToSourceDefinition("identifier", "end");
+verify.goToSourceDefinition("moduleSpecifier", { file: "/a.js" })
\ No newline at end of file
diff --git a/tests/cases/fourslash/server/goToSource2_nodeModulesWithTypes.ts b/tests/cases/fourslash/server/goToSource2_nodeModulesWithTypes.ts
new file mode 100644
index 0000000000000..a2166786c54cd
--- /dev/null
+++ b/tests/cases/fourslash/server/goToSource2_nodeModulesWithTypes.ts
@@ -0,0 +1,18 @@
+/// <reference path="../fourslash.ts" />
+
+// @moduleResolution: node
+
+// @Filename: /node_modules/foo/package.json
+//// { "name": "foo", "version": "1.0.0", "main": "./lib/main.js", "types": "./types/main.d.ts" }
+
+// @Filename: /node_modules/foo/lib/main.js
+//// export const /*end*/a = "a";
+
+// @Filename: /node_modules/foo/types/main.d.ts
+//// export declare const a: string;
+
+// @Filename: /index.ts
+//// import { a } from "foo";
+//// [|a/*start*/|]
+
+verify.goToSourceDefinition("start", "end");
diff --git a/tests/cases/fourslash/server/goToSource3_nodeModulesAtTypes.ts b/tests/cases/fourslash/server/goToSource3_nodeModulesAtTypes.ts
new file mode 100644
index 0000000000000..152d411040a12
--- /dev/null
+++ b/tests/cases/fourslash/server/goToSource3_nodeModulesAtTypes.ts
@@ -0,0 +1,21 @@
+/// <reference path="../fourslash.ts" />
+
+// @moduleResolution: node
+
+// @Filename: /node_modules/foo/package.json
+//// { "name": "foo", "version": "1.0.0", "main": "./lib/main.js" }
+
+// @Filename: /node_modules/foo/lib/main.js
+//// export const /*end*/a = "a";
+
+// @Filename: /node_modules/@types/foo/package.json
+//// { "name": "@types/foo", "version": "1.0.0", "types": "./index.d.ts" }
+
+// @Filename: /node_modules/@types/foo/index.d.ts
+//// export declare const a: string;
+
+// @Filename: /index.ts
+//// import { a } from "foo";
+//// [|a/*start*/|]
+
+verify.goToSourceDefinition("start", "end");
diff --git a/tests/cases/fourslash/server/goToSource5_sameAsGoToDef1.ts b/tests/cases/fourslash/server/goToSource5_sameAsGoToDef1.ts
new file mode 100644
index 0000000000000..f2b7e2b73457f
--- /dev/null
+++ b/tests/cases/fourslash/server/goToSource5_sameAsGoToDef1.ts
@@ -0,0 +1,17 @@
+/// <reference path="../fourslash.ts" />
+
+// @Filename: /a.ts
+//// export const /*end*/a = 'a';
+
+// @Filename: /a.d.ts
+//// export declare const a: string;
+
+// @Filename: /a.js
+//// export const a = 'a';
+
+// @Filename: /b.ts
+//// import { a } from './a';
+//// [|a/*start*/|]
+
+verify.goToDefinition("start", "end");
+verify.goToSourceDefinition("start", "end");
diff --git a/tests/cases/fourslash/server/goToSource6_sameAsGoToDef2.ts b/tests/cases/fourslash/server/goToSource6_sameAsGoToDef2.ts
new file mode 100644
index 0000000000000..87b7065a17b0a
--- /dev/null
+++ b/tests/cases/fourslash/server/goToSource6_sameAsGoToDef2.ts
@@ -0,0 +1,24 @@
+/// <reference path="../fourslash.ts" />
+
+// @Filename: /node_modules/foo/package.json
+//// { "name": "foo", "version": "1.2.3", "typesVersions": { "*": { "*": ["./types/*"] } } }
+
+// @Filename: /node_modules/foo/src/a.ts
+//// export const /*end*/a = 'a';
+
+// @Filename: /node_modules/foo/types/a.d.ts
+//// export declare const a: string;
+//// //# sourceMappingURL=a.d.ts.map
+
+// @Filename: /node_modules/foo/types/a.d.ts.map
+//// {"version":3,"file":"a.d.ts","sourceRoot":"","sources":["../src/a.ts"],"names":[],"mappings":"AAAA,eAAO,MAAM,EAAE,OAAO,CAAC;;AACvB,wBAAsB"}
+
+// @Filename: /node_modules/foo/dist/a.js
+//// export const a = 'a';
+
+// @Filename: /b.ts
+//// import { a } from 'foo/a';
+//// [|a/*start*/|]
+
+verify.goToDefinition("start", "end");
+verify.goToSourceDefinition("start", "end");
diff --git a/tests/cases/fourslash/server/goToSource7_conditionallyMinified.ts b/tests/cases/fourslash/server/goToSource7_conditionallyMinified.ts
new file mode 100644
index 0000000000000..6a0e13962ba38
--- /dev/null
+++ b/tests/cases/fourslash/server/goToSource7_conditionallyMinified.ts
@@ -0,0 +1,33 @@
+/// <reference path="../fourslash.ts" />
+
+// @moduleResolution: node
+
+// @Filename: /node_modules/react/package.json
+//// { "name": "react", "version": "16.8.6", "main": "index.js" }
+
+// @Filename: /node_modules/react/index.js
+//// 'use strict';
+//// 
+//// if (process.env.NODE_ENV === 'production') {
+////   module.exports = require('./cjs/react.production.min.js');
+//// } else {
+////   module.exports = require('./cjs/react.development.js');
+//// }
+
+// @Filename: /node_modules/react/cjs/react.production.min.js
+//// 'use strict';exports./*production*/useState=function(a){};exports.version='16.8.6';
+
+// @Filename: /node_modules/react/cjs/react.development.js
+//// 'use strict';
+//// if (process.env.NODE_ENV !== 'production') {
+////   (function() {
+////     function useState(initialState) {}
+////     exports./*development*/useState = useState;
+////     exports.version = '16.8.6';
+////   }());
+//// }
+
+// @Filename: /index.ts
+//// import { [|/*start*/useState|] } from 'react';
+
+verify.goToSourceDefinition("start", ["production", "development"]);
diff --git a/tests/cases/fourslash/server/goToSource8_mapFromAtTypes.ts b/tests/cases/fourslash/server/goToSource8_mapFromAtTypes.ts
new file mode 100644
index 0000000000000..690ae8b5fa5a6
--- /dev/null
+++ b/tests/cases/fourslash/server/goToSource8_mapFromAtTypes.ts
@@ -0,0 +1,80 @@
+/// <reference path="../fourslash.ts" />
+
+// @moduleResolution: node
+
+// @Filename: /node_modules/lodash/package.json
+//// { "name": "lodash", "version": "4.17.15", "main": "./lodash.js" }
+
+// @Filename: /node_modules/lodash/lodash.js
+//// ;(function() {
+////     /**
+////      * Adds two numbers.
+////      *
+////      * @static
+////      * @memberOf _
+////      * @since 3.4.0
+////      * @category Math
+////      * @param {number} augend The first number in an addition.
+////      * @param {number} addend The second number in an addition.
+////      * @returns {number} Returns the total.
+////      * @example
+////      *
+////      * _.add(6, 4);
+////      * // => 10
+////      */
+////     var [|/*variable*/add|] = createMathOperation(function(augend, addend) {
+////      return augend + addend;
+////     }, 0);
+////
+////     function lodash(value) {}
+////     lodash.[|/*property*/add|] = add;
+////
+////     /** Detect free variable `global` from Node.js. */
+////     var freeGlobal = typeof global == 'object' && global && global.Object === Object && global;
+////     /** Detect free variable `self`. */
+////     var freeSelf = typeof self == 'object' && self && self.Object === Object && self;
+////     /** Used as a reference to the global object. */
+////     var root = freeGlobal || freeSelf || Function('return this')();
+////     /** Detect free variable `exports`. */
+////     var freeExports = typeof exports == 'object' && exports && !exports.nodeType && exports;////     
+////     /** Detect free variable `module`. */
+////     var freeModule = freeExports && typeof module == 'object' && module && !module.nodeType && module;
+////     if (freeModule) {
+////       // Export for Node.js.
+////       (freeModule.exports = _)._ = _;
+////       // Export for CommonJS support.
+////       freeExports._ = _;
+////     }
+////     else {
+////       // Export to the global object.
+////       root._ = _;
+////     }
+//// }.call(this));
+
+// @Filename: /node_modules/@types/lodash/package.json
+//// { "name": "@types/lodash", "version": "4.14.97", "types": "index.d.ts" }
+
+// @Filename: /node_modules/@types/lodash/index.d.ts
+//// /// <reference path="./common/math.d.ts" />
+//// export = _;
+//// export as namespace _;
+//// declare const _: _.LoDashStatic;
+//// declare namespace _ {
+////     interface LoDashStatic {}
+//// }
+
+// @Filename: /node_modules/@types/lodash/common/math.d.ts
+//// import _ = require("../index");
+//// declare module "../index" {
+////     interface LoDashStatic {
+////         add(augend: number, addend: number): number;
+////     }
+//// }
+
+// @Filename: /index.ts
+//// import { [|/*start*/add|] } from 'lodash';
+
+verify.goToSourceDefinition("start", [
+  { marker: "variable", unverified: true },
+  { marker: "property", unverified: true },
+]);
diff --git a/tests/cases/fourslash/server/goToSource9_mapFromAtTypes2.ts b/tests/cases/fourslash/server/goToSource9_mapFromAtTypes2.ts
new file mode 100644
index 0000000000000..024fe099b4d8e
--- /dev/null
+++ b/tests/cases/fourslash/server/goToSource9_mapFromAtTypes2.ts
@@ -0,0 +1,81 @@
+/// <reference path="../fourslash.ts" />
+
+// @moduleResolution: node
+
+// @Filename: /node_modules/lodash/package.json
+//// { "name": "lodash", "version": "4.17.15", "main": "./lodash.js" }
+
+// @Filename: /node_modules/lodash/lodash.js
+//// ;(function() {
+////     /**
+////      * Adds two numbers.
+////      *
+////      * @static
+////      * @memberOf _
+////      * @since 3.4.0
+////      * @category Math
+////      * @param {number} augend The first number in an addition.
+////      * @param {number} addend The second number in an addition.
+////      * @returns {number} Returns the total.
+////      * @example
+////      *
+////      * _.add(6, 4);
+////      * // => 10
+////      */
+////     var [|/*variable*/add|] = createMathOperation(function(augend, addend) {
+////      return augend + addend;
+////     }, 0);
+////
+////     function lodash(value) {}
+////     lodash.[|/*property*/add|] = add;
+////
+////     /** Detect free variable `global` from Node.js. */
+////     var freeGlobal = typeof global == 'object' && global && global.Object === Object && global;
+////     /** Detect free variable `self`. */
+////     var freeSelf = typeof self == 'object' && self && self.Object === Object && self;
+////     /** Used as a reference to the global object. */
+////     var root = freeGlobal || freeSelf || Function('return this')();
+////     /** Detect free variable `exports`. */
+////     var freeExports = typeof exports == 'object' && exports && !exports.nodeType && exports;////     
+////     /** Detect free variable `module`. */
+////     var freeModule = freeExports && typeof module == 'object' && module && !module.nodeType && module;
+////     if (freeModule) {
+////       // Export for Node.js.
+////       (freeModule.exports = _)._ = _;
+////       // Export for CommonJS support.
+////       freeExports._ = _;
+////     }
+////     else {
+////       // Export to the global object.
+////       root._ = _;
+////     }
+//// }.call(this));
+
+// @Filename: /node_modules/@types/lodash/package.json
+//// { "name": "@types/lodash", "version": "4.14.97", "types": "index.d.ts" }
+
+// @Filename: /node_modules/@types/lodash/index.d.ts
+//// /// <reference path="./common/math.d.ts" />
+//// export = _;
+//// export as namespace _;
+//// declare const _: _.LoDashStatic;
+//// declare namespace _ {
+////     interface LoDashStatic {}
+//// }
+
+// @Filename: /node_modules/@types/lodash/common/math.d.ts
+//// import _ = require("../index");
+//// declare module "../index" {
+////     interface LoDashStatic {
+////         add(augend: number, addend: number): number;
+////     }
+//// }
+
+// @Filename: /index.ts
+//// import [|/*defaultImport*/_|], { [|/*unresolvableNamedImport*/foo|] } from [|/*moduleSpecifier*/'lodash'|];
+//// _.[|/*propertyAccess*/add|]
+
+verify.goToSourceDefinition("defaultImport", { file: "/node_modules/lodash/lodash.js", unverified: true });
+verify.goToSourceDefinition("unresolvableNamedImport", { file: "/node_modules/lodash/lodash.js", unverified: true });
+verify.goToSourceDefinition("moduleSpecifier", { file: "/node_modules/lodash/lodash.js", unverified: true });
+verify.goToSourceDefinition("propertyAccess", [{ marker: "variable", unverified: true }, { marker: "property", unverified: true }])