show completion in destructured parameter if containing function was contextually typed

Author: vladima

src/services/services.ts

@@ -3504,7 +3504,11 @@ namespace ts {
                         // We don't want to complete using the type acquired by the shape
                         // of the binding pattern; we are only interested in types acquired
                         // through type declaration or inference.
-                        if (rootDeclaration.initializer || rootDeclaration.type) {
+                        // Also proceed if rootDeclaration is parameter and if its containing function expression\arrow function is contextually typed -
+                        // type of parameter will flow in from the contextual type of the function
+                        if (rootDeclaration.initializer ||
+                            rootDeclaration.type ||
+                            (rootDeclaration.kind === SyntaxKind.Parameter && isExpression(rootDeclaration.parent) && typeChecker.getContextualType(<Expression>rootDeclaration.parent))) {
                             typeForObject = typeChecker.getTypeAtLocation(objectLikeContainer);
                             existingMembers = (<BindingPattern>objectLikeContainer).elements;
                         }

tests/cases/fourslash/objectLiteralBindingInParameter.ts

@@ -0,0 +1,15 @@
+/// <reference path="fourslash.ts"/>
+
+////interface I { x1: number; x2: string }
+////function f(cb: (ev: I) => any) { }
+////f(({/*1*/}) => 0);
+
+////[<I>null].reduce(({/*2*/}, b) => b);
+
+goTo.marker("1");
+verify.completionListContains("x1");
+verify.completionListContains("x2");
+
+goTo.marker("2");
+verify.completionListContains("x1");
+verify.completionListContains("x2");