Merge pull request #12312 from Microsoft/widen-literal-types-of-param…

…eter-properties

Widen literal types of parameter properties

src/compiler/checker.ts

@@ -14744,7 +14744,7 @@ namespace ts {
         function checkDeclarationInitializer(declaration: VariableLikeDeclaration) {
             const type = checkExpressionCached(declaration.initializer);
             return getCombinedNodeFlags(declaration) & NodeFlags.Const ||
-                getCombinedModifierFlags(declaration) & ModifierFlags.Readonly ||
+                getCombinedModifierFlags(declaration) & ModifierFlags.Readonly && !isParameterPropertyDeclaration(declaration) ||
                 isTypeAssertion(declaration.initializer) ? type : getWidenedLiteralType(type);
         }
 

src/compiler/utilities.ts

@@ -4497,7 +4497,7 @@ namespace ts {
         }
     }
 
-    export function isParameterPropertyDeclaration(node: ParameterDeclaration): boolean {
+    export function isParameterPropertyDeclaration(node: Node): boolean {
         return hasModifier(node, ModifierFlags.ParameterPropertyModifier) && node.parent.kind === SyntaxKind.Constructor && isClassLike(node.parent.parent);
     }
 

tests/baselines/reference/literalTypesWidenInParameterPosition.errors.txt

@@ -0,0 +1,15 @@
+tests/cases/conformance/types/literal/literalTypesWidenInParameterPosition.ts(4,9): error TS2322: Type '5' is not assignable to type '1'.
+
+
+==== tests/cases/conformance/types/literal/literalTypesWidenInParameterPosition.ts (1 errors) ====
+    class D {
+        readonly noWiden = 1
+        constructor(readonly widen = 2) {
+            this.noWiden = 5; // error
+            ~~~~~~~~~~~~
+!!! error TS2322: Type '5' is not assignable to type '1'.
+            this.widen = 6; // ok
+        }
+    }
+    new D(7); // ok
+

tests/baselines/reference/literalTypesWidenInParameterPosition.js

@@ -0,0 +1,23 @@
+//// [literalTypesWidenInParameterPosition.ts]
+class D {
+    readonly noWiden = 1
+    constructor(readonly widen = 2) {
+        this.noWiden = 5; // error
+        this.widen = 6; // ok
+    }
+}
+new D(7); // ok
+
+
+//// [literalTypesWidenInParameterPosition.js]
+var D = (function () {
+    function D(widen) {
+        if (widen === void 0) { widen = 2; }
+        this.widen = widen;
+        this.noWiden = 1;
+        this.noWiden = 5; // error
+        this.widen = 6; // ok
+    }
+    return D;
+}());
+new D(7); // ok

tests/cases/conformance/types/literal/literalTypesWidenInParameterPosition.ts

@@ -0,0 +1,8 @@
+class D {
+    readonly noWiden = 1
+    constructor(readonly widen = 2) {
+        this.noWiden = 5; // error
+        this.widen = 6; // ok
+    }
+}
+new D(7); // ok