From 2a20e915e8850a47e059f5c1d12f866394ebd5e8 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Mon, 18 Jan 2016 16:58:09 -0800
Subject: [PATCH] Allow assignments to readonly properties in constructors

---
 src/compiler/checker.ts | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 75751b45458..9fcf951ce83 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -10366,6 +10366,17 @@ namespace ts {
             return true;
         }
 
+        function isReferenceWithinOwnConstructor(expr: Expression, symbol: Symbol): boolean {
+            // Allow assignments to readonly properties or methods (but not readonly accessors) within constructors
+            // of the same class declaration.
+            if ((expr.kind === SyntaxKind.PropertyAccessExpression || expr.kind === SyntaxKind.ElementAccessExpression) &&
+                (expr as PropertyAccessExpression | ElementAccessExpression).expression.kind === SyntaxKind.ThisKeyword &&
+                symbol.flags & (SymbolFlags.Property | SymbolFlags.Method)) {
+                const func = getContainingFunction(expr);
+                return func && func.kind === SyntaxKind.Constructor && func.parent === symbol.valueDeclaration.parent;
+            }
+        }
+
         function isReadonlySymbol(symbol: Symbol): boolean {
             if (symbol.flags & (SymbolFlags.Property | SymbolFlags.Method)) {
                 return symbol === undefinedSymbol || (getDeclarationFlagsFromSymbol(symbol) & NodeFlags.Readonly) !== 0;
@@ -10400,7 +10411,7 @@ namespace ts {
                         error(expr, invalidReferenceMessage);
                         return false;
                     }
-                    if (isReadonlySymbol(symbol) || isConstantSymbol(symbol)) {
+                    if (isConstantSymbol(symbol) || (isReadonlySymbol(symbol) && !isReferenceWithinOwnConstructor(expr, symbol))) {
                         error(expr, constantVariableMessage);
                         return false;
                     }