Merge pull request #8712 from Microsoft/readonly_assignment_in_constructor

Allow assignment to readonly parameter property within the constructor

src/compiler/checker.ts

@@ -11877,8 +11877,14 @@ namespace ts {
                 if (symbol.flags & SymbolFlags.Property &&
                     (expr.kind === SyntaxKind.PropertyAccessExpression || expr.kind === SyntaxKind.ElementAccessExpression) &&
                     (expr as PropertyAccessExpression | ElementAccessExpression).expression.kind === SyntaxKind.ThisKeyword) {
+                    // Look for if this is the constructor for the class that `symbol` is a property of.
                     const func = getContainingFunction(expr);
-                    return !(func && func.kind === SyntaxKind.Constructor && func.parent === symbol.valueDeclaration.parent);
+                    if (!(func && func.kind === SyntaxKind.Constructor))
+                        return true;
+                    // If func.parent is a class and symbol is a (readonly) property of that class, or
+                    // if func is a constructor and symbol is a (readonly) parameter property declared in it,
+                    // then symbol is writeable here.
+                    return !(func.parent === symbol.valueDeclaration.parent || func === symbol.valueDeclaration.parent);
                 }
                 return true;
             }

tests/baselines/reference/readonlyConstructorAssignment.errors.txt

@@ -0,0 +1,50 @@
+tests/cases/conformance/classes/constructorDeclarations/constructorParameters/readonlyConstructorAssignment.ts(13,9): error TS2450: Left-hand side of assignment expression cannot be a constant or a read-only property.
+tests/cases/conformance/classes/constructorDeclarations/constructorParameters/readonlyConstructorAssignment.ts(33,7): error TS2415: Class 'E' incorrectly extends base class 'D'.
+  Property 'x' is private in type 'D' but not in type 'E'.
+
+
+==== tests/cases/conformance/classes/constructorDeclarations/constructorParameters/readonlyConstructorAssignment.ts (2 errors) ====
+    // Tests that readonly parameter properties behave like regular readonly properties
+    
+    class A {
+        constructor(readonly x: number) {
+            this.x = 0;
+        }
+    }
+    
+    class B extends A {
+        constructor(x: number) {
+            super(x);
+            // Fails, x is readonly
+            this.x = 1;
+            ~~~~~~
+!!! error TS2450: Left-hand side of assignment expression cannot be a constant or a read-only property.
+        }
+    }
+    
+    class C extends A {
+        // This is the usual behavior of readonly properties:
+        // if one is redeclared in a base class, then it can be assigned to.
+        constructor(readonly x: number) {
+            super(x);
+            this.x = 1;
+        }
+    }
+    
+    class D {
+        constructor(private readonly x: number) {
+            this.x = 0;
+        }
+    }
+    
+    // Fails, can't redeclare readonly property
+    class E extends D {
+          ~
+!!! error TS2415: Class 'E' incorrectly extends base class 'D'.
+!!! error TS2415:   Property 'x' is private in type 'D' but not in type 'E'.
+        constructor(readonly x: number) {
+            super(x);
+            this.x = 1;
+        }
+    }
+    

tests/baselines/reference/readonlyConstructorAssignment.js

@@ -0,0 +1,92 @@
+//// [readonlyConstructorAssignment.ts]
+// Tests that readonly parameter properties behave like regular readonly properties
+
+class A {
+    constructor(readonly x: number) {
+        this.x = 0;
+    }
+}
+
+class B extends A {
+    constructor(x: number) {
+        super(x);
+        // Fails, x is readonly
+        this.x = 1;
+    }
+}
+
+class C extends A {
+    // This is the usual behavior of readonly properties:
+    // if one is redeclared in a base class, then it can be assigned to.
+    constructor(readonly x: number) {
+        super(x);
+        this.x = 1;
+    }
+}
+
+class D {
+    constructor(private readonly x: number) {
+        this.x = 0;
+    }
+}
+
+// Fails, can't redeclare readonly property
+class E extends D {
+    constructor(readonly x: number) {
+        super(x);
+        this.x = 1;
+    }
+}
+
+
+//// [readonlyConstructorAssignment.js]
+// Tests that readonly parameter properties behave like regular readonly properties
+var __extends = (this && this.__extends) || function (d, b) {
+    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
+    function __() { this.constructor = d; }
+    d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
+};
+var A = (function () {
+    function A(x) {
+        this.x = x;
+        this.x = 0;
+    }
+    return A;
+}());
+var B = (function (_super) {
+    __extends(B, _super);
+    function B(x) {
+        _super.call(this, x);
+        // Fails, x is readonly
+        this.x = 1;
+    }
+    return B;
+}(A));
+var C = (function (_super) {
+    __extends(C, _super);
+    // This is the usual behavior of readonly properties:
+    // if one is redeclared in a base class, then it can be assigned to.
+    function C(x) {
+        _super.call(this, x);
+        this.x = x;
+        this.x = 1;
+    }
+    return C;
+}(A));
+var D = (function () {
+    function D(x) {
+        this.x = x;
+        this.x = 0;
+    }
+    return D;
+}());
+// Fails, can't redeclare readonly property
+var E = (function (_super) {
+    __extends(E, _super);
+    function E(x) {
+        _super.call(this, x);
+        this.x = x;
+        this.x = 1;
+    }
+    return E;
+}(D));

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

@@ -0,0 +1,38 @@
+// Tests that readonly parameter properties behave like regular readonly properties
+
+class A {
+    constructor(readonly x: number) {
+        this.x = 0;
+    }
+}
+
+class B extends A {
+    constructor(x: number) {
+        super(x);
+        // Fails, x is readonly
+        this.x = 1;
+    }
+}
+
+class C extends A {
+    // This is the usual behavior of readonly properties:
+    // if one is redeclared in a base class, then it can be assigned to.
+    constructor(readonly x: number) {
+        super(x);
+        this.x = 1;
+    }
+}
+
+class D {
+    constructor(private readonly x: number) {
+        this.x = 0;
+    }
+}
+
+// Fails, can't redeclare readonly property
+class E extends D {
+    constructor(readonly x: number) {
+        super(x);
+        this.x = 1;
+    }
+}