Allow assignment to readonly parameter property within the constructor

src/compiler/checker.ts

@@ -11882,8 +11882,18 @@ namespace ts {
                 if (symbol.flags & SymbolFlags.Property &&
                     (expr.kind === SyntaxKind.PropertyAccessExpression || expr.kind === SyntaxKind.ElementAccessExpression) &&
                     (expr as PropertyAccessExpression | ElementAccessExpression).expression.kind === SyntaxKind.ThisKeyword) {
-                    const func = getContainingFunction(expr);
-                    return !(func && func.kind === SyntaxKind.Constructor && func.parent === symbol.valueDeclaration.parent);
+                    return !isInConstructor(getContainingFunction(expr));
+                    function isInConstructor(func: FunctionLikeDeclaration) {
+                        if (!func)
+                            return false;
+                        if (func.kind !== SyntaxKind.Constructor)
+                            return false;
+                        if (func.parent === symbol.valueDeclaration.parent) // If the symbol was declared in the class:
+                            return true;
+                        if (func === symbol.valueDeclaration.parent) // If symbolDecl is a parameter property of the constructor:
+                            return true;
+                        return false;
+                    }
                 }
                 return true;
             }

tests/baselines/reference/readonlyConstructorAssignment.js

@@ -0,0 +1,16 @@
+//// [readonlyConstructorAssignment.ts]
+class A {
+    constructor(readonly x: number) {
+        this.x = 7;
+    }
+}
+
+
+//// [readonlyConstructorAssignment.js]
+var A = (function () {
+    function A(x) {
+        this.x = x;
+        this.x = 7;
+    }
+    return A;
+}());

tests/baselines/reference/readonlyConstructorAssignment.symbols

@@ -0,0 +1,14 @@
+=== tests/cases/conformance/classes/constructorDeclarations/constructorParameters/readonlyConstructorAssignment.ts ===
+class A {
+>A : Symbol(A, Decl(readonlyConstructorAssignment.ts, 0, 0))
+
+    constructor(readonly x: number) {
+>x : Symbol(A.x, Decl(readonlyConstructorAssignment.ts, 1, 16))
+
+        this.x = 7;
+>this.x : Symbol(A.x, Decl(readonlyConstructorAssignment.ts, 1, 16))
+>this : Symbol(A, Decl(readonlyConstructorAssignment.ts, 0, 0))
+>x : Symbol(A.x, Decl(readonlyConstructorAssignment.ts, 1, 16))
+    }
+}
+

tests/baselines/reference/readonlyConstructorAssignment.types

@@ -0,0 +1,16 @@
+=== tests/cases/conformance/classes/constructorDeclarations/constructorParameters/readonlyConstructorAssignment.ts ===
+class A {
+>A : A
+
+    constructor(readonly x: number) {
+>x : number
+
+        this.x = 7;
+>this.x = 7 : number
+>this.x : number
+>this : this
+>x : number
+>7 : number
+    }
+}
+

tests/cases/conformance/classes/constructorDeclarations/constructorParameters/readonlyConstructorAssignment.ts

@@ -0,0 +1,5 @@
+class A {
+    constructor(readonly x: number) {
+        this.x = 7;
+    }
+}