If resolvedFileName differs with realPath only in casing use the resolvedFileName before realpath so that errors can be reported with forceConsistentCasingInFileNames

src/compiler/moduleNameResolver.ts

@@ -392,10 +392,12 @@ namespace ts {
         if (resolved) {
             const { fileName, packageId } = resolved;
             const resolvedFileName = options.preserveSymlinks ? fileName : realPath(fileName, host, traceEnabled);
+            const pathsAreEqual = arePathsEqual(fileName, resolvedFileName, host);
             resolvedTypeReferenceDirective = {
                 primary,
-                resolvedFileName,
-                originalPath: arePathsEqual(fileName, resolvedFileName, host) ? undefined : fileName,
+                // If the fileName and realpath are differing only in casing prefer fileName so that we can issue correct errors for casing under forceConsistentCasingInFileNames
+                resolvedFileName: pathsAreEqual ? fileName : resolvedFileName,
+                originalPath: pathsAreEqual ? undefined : fileName,
                 packageId,
                 isExternalLibraryImport: pathContainsNodeModules(fileName),
             };
@@ -1406,8 +1408,10 @@ namespace ts {
                 let resolvedValue = resolved.value;
                 if (!compilerOptions.preserveSymlinks && resolvedValue && !resolvedValue.originalPath) {
                     const path = realPath(resolvedValue.path, host, traceEnabled);
-                    const originalPath = arePathsEqual(path, resolvedValue.path, host) ? undefined : resolvedValue.path;
-                    resolvedValue = { ...resolvedValue, path, originalPath };
+                    const pathsAreEqual = arePathsEqual(path, resolvedValue.path, host);
+                    const originalPath = pathsAreEqual ? undefined : resolvedValue.path;
+                    // If the path and realpath are differing only in casing prefer path so that we can issue correct errors for casing under forceConsistentCasingInFileNames
+                    resolvedValue = { ...resolvedValue, path: pathsAreEqual ? resolvedValue.path : path, originalPath };
                 }
                 // For node_modules lookups, get the real path so that multiple accesses to an `npm link`-ed module do not create duplicate files.
                 return { value: resolvedValue && { resolved: resolvedValue, isExternalLibraryImport: true } };

src/testRunner/tsconfig.json

@@ -151,6 +151,7 @@
         "unittests/tsc/cancellationToken.ts",
         "unittests/tsc/composite.ts",
         "unittests/tsc/declarationEmit.ts",
+        "unittests/tsc/forceConsistentCasingInFileNames.ts",
         "unittests/tsc/incremental.ts",
         "unittests/tsc/listFilesOnly.ts",
         "unittests/tsc/projectReferences.ts",

src/testRunner/unittests/tsc/forceConsistentCasingInFileNames.ts

@@ -0,0 +1,18 @@
+namespace ts {
+    describe("unittests:: tsc:: forceConsistentCasingInFileNames::", () => {
+        verifyTsc({
+            scenario: "forceConsistentCasingInFileNames",
+            subScenario: "with relative and non relative file resolutions",
+            commandLineArgs: ["/src/project/src/struct.d.ts", "--forceConsistentCasingInFileNames", "--explainFiles"],
+            fs: () => loadProjectFromFiles({
+                "/src/project/src/struct.d.ts": Utils.dedent`
+                    import * as xs1 from "fp-ts/lib/Struct";
+                    import * as xs2 from "fp-ts/lib/struct";
+                    import * as xs3 from "./Struct";
+                    import * as xs4 from "./struct";
+                `,
+                "/src/project/node_modules/fp-ts/lib/struct.d.ts": `export function foo(): void`,
+            }),
+        });
+    });
+}

src/testRunner/unittests/tscWatch/forceConsistentCasingInFileNames.ts

@@ -113,7 +113,7 @@ export const Fragment: unique symbol;
                     path: `${projectRoot}/tsconfig.json`,
                     content: JSON.stringify({
                         compilerOptions: { jsx: "react-jsx", jsxImportSource: "react", forceConsistentCasingInFileNames: true },
-                        files: ["node_modules/react/jsx-Runtime/index.d.ts", "index.tsx"] // NB: casing does not match disk
+                        files: ["node_modules/react/Jsx-Runtime/index.d.ts", "index.tsx"]
                     })
                 }
             ], { currentDirectory: projectRoot }),

tests/baselines/reference/tsc/forceConsistentCasingInFileNames/with-relative-and-non-relative-file-resolutions.js

@@ -0,0 +1,72 @@
+Input::
+//// [/lib/lib.d.ts]
+/// <reference no-default-lib="true"/>
+interface Boolean {}
+interface Function {}
+interface CallableFunction {}
+interface NewableFunction {}
+interface IArguments {}
+interface Number { toExponential: any; }
+interface Object {}
+interface RegExp {}
+interface String { charAt: any; }
+interface Array<T> { length: number; [n: number]: T; }
+interface ReadonlyArray<T> {}
+declare const console: { log(msg: any): void; };
+
+//// [/src/project/node_modules/fp-ts/lib/struct.d.ts]
+export function foo(): void
+
+//// [/src/project/src/struct.d.ts]
+import * as xs1 from "fp-ts/lib/Struct";
+import * as xs2 from "fp-ts/lib/struct";
+import * as xs3 from "./Struct";
+import * as xs4 from "./struct";
+
+
+
+
+Output::
+/lib/tsc /src/project/src/struct.d.ts --forceConsistentCasingInFileNames --explainFiles
+src/project/src/struct.d.ts:2:22 - error TS1149: File name '/src/project/node_modules/fp-ts/lib/struct.d.ts' differs from already included file name '/src/project/node_modules/fp-ts/lib/Struct.d.ts' only in casing.
+  The file is in the program because:
+    Imported via "fp-ts/lib/Struct" from file '/src/project/src/struct.d.ts'
+    Imported via "fp-ts/lib/struct" from file '/src/project/src/struct.d.ts'
+
+2 import * as xs2 from "fp-ts/lib/struct";
+                       ~~~~~~~~~~~~~~~~~~
+
+  src/project/src/struct.d.ts:1:22
+    1 import * as xs1 from "fp-ts/lib/Struct";
+                           ~~~~~~~~~~~~~~~~~~
+    File is included via import here.
+
+src/project/src/struct.d.ts:3:22 - error TS1149: File name '/src/project/src/Struct.d.ts' differs from already included file name '/src/project/src/struct.d.ts' only in casing.
+  The file is in the program because:
+    Root file specified for compilation
+    Imported via "./Struct" from file '/src/project/src/struct.d.ts'
+    Imported via "./struct" from file '/src/project/src/struct.d.ts'
+
+3 import * as xs3 from "./Struct";
+                       ~~~~~~~~~~
+
+  src/project/src/struct.d.ts:4:22
+    4 import * as xs4 from "./struct";
+                           ~~~~~~~~~~
+    File is included via import here.
+
+lib/lib.d.ts
+  Default library for target 'es3'
+src/project/node_modules/fp-ts/lib/Struct.d.ts
+  Imported via "fp-ts/lib/Struct" from file 'src/project/src/struct.d.ts'
+  Imported via "fp-ts/lib/struct" from file 'src/project/src/struct.d.ts'
+src/project/src/struct.d.ts
+  Root file specified for compilation
+  Imported via "./Struct" from file 'src/project/src/struct.d.ts'
+  Imported via "./struct" from file 'src/project/src/struct.d.ts'
+
+Found 2 errors in the same file, starting at: src/project/src/struct.d.ts:2
+
+exitCode:: ExitStatus.DiagnosticsPresent_OutputsGenerated
+
+

tests/baselines/reference/tscWatch/forceConsistentCasingInFileNames/jsxImportSource-option-changed.js

@@ -33,27 +33,27 @@ export const Fragment: unique symbol;
 export const App = () => <div propA={true}></div>;
 
 //// [/user/username/projects/myproject/tsconfig.json]
-{"compilerOptions":{"jsx":"react-jsx","jsxImportSource":"react","forceConsistentCasingInFileNames":true},"files":["node_modules/react/jsx-Runtime/index.d.ts","index.tsx"]}
+{"compilerOptions":{"jsx":"react-jsx","jsxImportSource":"react","forceConsistentCasingInFileNames":true},"files":["node_modules/react/Jsx-Runtime/index.d.ts","index.tsx"]}
 
 
 /a/lib/tsc.js --w --p . --explainFiles
 Output::
 >> Screen clear
 [12:00:31 AM] Starting compilation in watch mode...
 
-error TS1149: File name '/user/username/projects/myproject/node_modules/react/Jsx-runtime/index.d.ts' differs from already included file name '/user/username/projects/myproject/node_modules/react/jsx-Runtime/index.d.ts' only in casing.
+error TS1149: File name '/user/username/projects/myproject/node_modules/react/jsx-runtime/index.d.ts' differs from already included file name '/user/username/projects/myproject/node_modules/react/Jsx-Runtime/index.d.ts' only in casing.
   The file is in the program because:
     Part of 'files' list in tsconfig.json
     Imported via "react/jsx-runtime" from file '/user/username/projects/myproject/index.tsx' with packageId 'react/jsx-runtime/index.d.ts@0.0.1' to import 'jsx' and 'jsxs' factory functions
 
   tsconfig.json:1:115
-    1 {"compilerOptions":{"jsx":"react-jsx","jsxImportSource":"react","forceConsistentCasingInFileNames":true},"files":["node_modules/react/jsx-Runtime/index.d.ts","index.tsx"]}
+    1 {"compilerOptions":{"jsx":"react-jsx","jsxImportSource":"react","forceConsistentCasingInFileNames":true},"files":["node_modules/react/Jsx-Runtime/index.d.ts","index.tsx"]}
                                                                                                                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     File is matched by 'files' list specified here.
 
 ../../../../a/lib/lib.d.ts
   Default library for target 'es3'
-node_modules/react/jsx-Runtime/index.d.ts
+node_modules/react/Jsx-Runtime/index.d.ts
   Part of 'files' list in tsconfig.json
   Imported via "react/jsx-runtime" from file 'index.tsx' with packageId 'react/jsx-runtime/index.d.ts@0.0.1' to import 'jsx' and 'jsxs' factory functions
 index.tsx
@@ -62,12 +62,12 @@ index.tsx
 
 
 
-Program root files: ["/user/username/projects/myproject/node_modules/react/jsx-Runtime/index.d.ts","/user/username/projects/myproject/index.tsx"]
+Program root files: ["/user/username/projects/myproject/node_modules/react/Jsx-Runtime/index.d.ts","/user/username/projects/myproject/index.tsx"]
 Program options: {"jsx":4,"jsxImportSource":"react","forceConsistentCasingInFileNames":true,"watch":true,"project":"/user/username/projects/myproject","explainFiles":true,"configFilePath":"/user/username/projects/myproject/tsconfig.json"}
 Program structureReused: Not
 Program files::
 /a/lib/lib.d.ts
-/user/username/projects/myproject/node_modules/react/jsx-Runtime/index.d.ts
+/user/username/projects/myproject/node_modules/react/Jsx-Runtime/index.d.ts
 /user/username/projects/myproject/index.tsx
 
 No cached semantic diagnostics in the builder::
@@ -81,7 +81,7 @@ WatchedFiles::
 /user/username/projects/myproject/tsconfig.json:
   {"fileName":"/user/username/projects/myproject/tsconfig.json","pollingInterval":250}
 /user/username/projects/myproject/node_modules/react/jsx-runtime/index.d.ts:
-  {"fileName":"/user/username/projects/myproject/node_modules/react/jsx-Runtime/index.d.ts","pollingInterval":250}
+  {"fileName":"/user/username/projects/myproject/node_modules/react/Jsx-Runtime/index.d.ts","pollingInterval":250}
 /user/username/projects/myproject/index.tsx:
   {"fileName":"/user/username/projects/myproject/index.tsx","pollingInterval":250}
 /a/lib/lib.d.ts: