From e3a0687327128c4cce55b8ce448ce8ad7d3fca9b Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Fri, 17 Feb 2017 06:56:58 -0800
Subject: [PATCH] Contextual this in 'obj.xxx = function(...)' or 'obj[xxx] =
 function(...)'

---
 src/compiler/checker.ts | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 3208d303ab0..9fb71884c51 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -11518,6 +11518,14 @@ namespace ts {
                     // for 'this' is the type of the object literal itself.
                     return checkExpressionCached(containingLiteral);
                 }
+                // In an assignment of the form 'obj.xxx = function(...)' or 'obj[xxx] = function(...)', the
+                // contextual type for 'this' is 'obj'.
+                if (func.parent.kind === SyntaxKind.BinaryExpression && (<BinaryExpression>func.parent).operatorToken.kind === SyntaxKind.EqualsToken) {
+                    const target = (<BinaryExpression>func.parent).left;
+                    if (target.kind === SyntaxKind.PropertyAccessExpression || target.kind === SyntaxKind.ElementAccessExpression) {
+                        return checkExpressionCached((<PropertyAccessExpression | ElementAccessExpression>target).expression);
+                    }
+                }
             }
             return undefined;
         }