microsoft · ahejlsberg · Sep 24, 2015 · Sep 23, 2015 · Sep 24, 2015 · Sep 24, 2015
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -2293,10 +2293,17 @@ namespace ts {
             return type && (type.flags & TypeFlags.Any) !== 0;
         }
 
+        // Return the type of a binding element parent. We check SymbolLinks first to see if a type has been
+        // assigned by contextual typing.
+        function getTypeForBindingElementParent(node: VariableLikeDeclaration) {
+            let symbol = getSymbolOfNode(node);
+            return symbol && getSymbolLinks(symbol).type || getTypeForVariableLikeDeclaration(node);
+        }
+
         // Return the inferred type for a binding element
         function getTypeForBindingElement(declaration: BindingElement): Type {
             let pattern = <BindingPattern>declaration.parent;
-            let parentType = getTypeForVariableLikeDeclaration(<VariableLikeDeclaration>pattern.parent);
+            let parentType = getTypeForBindingElementParent(<VariableLikeDeclaration>pattern.parent);
             // If parent has the unknown (error) type, then so does this binding element
             if (parentType === unknownType) {
                 return unknownType;
@@ -9162,10 +9169,24 @@ namespace ts {
             }
         }
 
+        // When contextual typing assigns a type to a parameter that contains a binding pattern, we also need to push
+        // the destructured type into the contained binding elements.
+        function assignBindingElementTypes(node: VariableLikeDeclaration) {
+            if (isBindingPattern(node.name)) {
+                for (let element of (<BindingPattern>node.name).elements) {
+                    if (element.kind !== SyntaxKind.OmittedExpression) {
+                        getSymbolLinks(getSymbolOfNode(element)).type = getTypeForBindingElement(element);
+                        assignBindingElementTypes(element);
+                    }
+                }
+            }
+        }
+
         function assignTypeToParameterAndFixTypeParameters(parameter: Symbol, contextualType: Type, mapper: TypeMapper) {
             let links = getSymbolLinks(parameter);
             if (!links.type) {
                 links.type = instantiateType(contextualType, mapper);
+                assignBindingElementTypes(<ParameterDeclaration>parameter.valueDeclaration);
             }
             else if (isInferentialContext(mapper)) {
                 // Even if the parameter already has a type, it might be because it was given a type while

diff --git a/tests/baselines/reference/destructuringWithGenericParameter.js b/tests/baselines/reference/destructuringWithGenericParameter.js
@@ -0,0 +1,30 @@
+//// [destructuringWithGenericParameter.ts]
+class GenericClass<T> {
+    payload: T;
+}
+
+var genericObject = new GenericClass<{ greeting: string }>();
+
+function genericFunction<T>(object: GenericClass<T>, callback: (payload: T) => void) {
+    callback(object.payload);
+}
+
+genericFunction(genericObject, ({greeting}) => {
+    var s = greeting.toLocaleLowerCase();  // Greeting should be of type string
+});
+
+
+//// [destructuringWithGenericParameter.js]
+var GenericClass = (function () {
+    function GenericClass() {
+    }
+    return GenericClass;
+})();
+var genericObject = new GenericClass();
+function genericFunction(object, callback) {
+    callback(object.payload);
+}
+genericFunction(genericObject, function (_a) {
+    var greeting = _a.greeting;
+    var s = greeting.toLocaleLowerCase(); // Greeting should be of type string
+});
diff --git a/tests/baselines/reference/destructuringWithGenericParameter.symbols b/tests/baselines/reference/destructuringWithGenericParameter.symbols
@@ -0,0 +1,45 @@
+=== tests/cases/compiler/destructuringWithGenericParameter.ts ===
+class GenericClass<T> {
+>GenericClass : Symbol(GenericClass, Decl(destructuringWithGenericParameter.ts, 0, 0))
+>T : Symbol(T, Decl(destructuringWithGenericParameter.ts, 0, 19))
+
+    payload: T;
+>payload : Symbol(payload, Decl(destructuringWithGenericParameter.ts, 0, 23))
+>T : Symbol(T, Decl(destructuringWithGenericParameter.ts, 0, 19))
+}
+
+var genericObject = new GenericClass<{ greeting: string }>();
+>genericObject : Symbol(genericObject, Decl(destructuringWithGenericParameter.ts, 4, 3))
+>GenericClass : Symbol(GenericClass, Decl(destructuringWithGenericParameter.ts, 0, 0))
+>greeting : Symbol(greeting, Decl(destructuringWithGenericParameter.ts, 4, 38))
+
+function genericFunction<T>(object: GenericClass<T>, callback: (payload: T) => void) {
+>genericFunction : Symbol(genericFunction, Decl(destructuringWithGenericParameter.ts, 4, 61))
+>T : Symbol(T, Decl(destructuringWithGenericParameter.ts, 6, 25))
+>object : Symbol(object, Decl(destructuringWithGenericParameter.ts, 6, 28))
+>GenericClass : Symbol(GenericClass, Decl(destructuringWithGenericParameter.ts, 0, 0))
+>T : Symbol(T, Decl(destructuringWithGenericParameter.ts, 6, 25))
+>callback : Symbol(callback, Decl(destructuringWithGenericParameter.ts, 6, 52))
+>payload : Symbol(payload, Decl(destructuringWithGenericParameter.ts, 6, 64))
+>T : Symbol(T, Decl(destructuringWithGenericParameter.ts, 6, 25))
+
+    callback(object.payload);
+>callback : Symbol(callback, Decl(destructuringWithGenericParameter.ts, 6, 52))
+>object.payload : Symbol(GenericClass.payload, Decl(destructuringWithGenericParameter.ts, 0, 23))
+>object : Symbol(object, Decl(destructuringWithGenericParameter.ts, 6, 28))
+>payload : Symbol(GenericClass.payload, Decl(destructuringWithGenericParameter.ts, 0, 23))
+}
+
+genericFunction(genericObject, ({greeting}) => {
+>genericFunction : Symbol(genericFunction, Decl(destructuringWithGenericParameter.ts, 4, 61))
+>genericObject : Symbol(genericObject, Decl(destructuringWithGenericParameter.ts, 4, 3))
+>greeting : Symbol(greeting, Decl(destructuringWithGenericParameter.ts, 10, 33))
+
+    var s = greeting.toLocaleLowerCase();  // Greeting should be of type string
+>s : Symbol(s, Decl(destructuringWithGenericParameter.ts, 11, 7))
+>greeting.toLocaleLowerCase : Symbol(String.toLocaleLowerCase, Decl(lib.d.ts, 402, 26))
+>greeting : Symbol(greeting, Decl(destructuringWithGenericParameter.ts, 10, 33))
+>toLocaleLowerCase : Symbol(String.toLocaleLowerCase, Decl(lib.d.ts, 402, 26))
+
+});
+
diff --git a/tests/baselines/reference/destructuringWithGenericParameter.types b/tests/baselines/reference/destructuringWithGenericParameter.types
@@ -0,0 +1,50 @@
+=== tests/cases/compiler/destructuringWithGenericParameter.ts ===
+class GenericClass<T> {
+>GenericClass : GenericClass<T>
+>T : T
+
+    payload: T;
+>payload : T
+>T : T
+}
+
+var genericObject = new GenericClass<{ greeting: string }>();
+>genericObject : GenericClass<{ greeting: string; }>
+>new GenericClass<{ greeting: string }>() : GenericClass<{ greeting: string; }>
+>GenericClass : typeof GenericClass
+>greeting : string
+
+function genericFunction<T>(object: GenericClass<T>, callback: (payload: T) => void) {
+>genericFunction : <T>(object: GenericClass<T>, callback: (payload: T) => void) => void
+>T : T
+>object : GenericClass<T>
+>GenericClass : GenericClass<T>
+>T : T
+>callback : (payload: T) => void
+>payload : T
+>T : T
+
+    callback(object.payload);
+>callback(object.payload) : void
+>callback : (payload: T) => void
+>object.payload : T
+>object : GenericClass<T>
+>payload : T
+}
+
+genericFunction(genericObject, ({greeting}) => {
+>genericFunction(genericObject, ({greeting}) => {    var s = greeting.toLocaleLowerCase();  // Greeting should be of type string}) : void
+>genericFunction : <T>(object: GenericClass<T>, callback: (payload: T) => void) => void
+>genericObject : GenericClass<{ greeting: string; }>
+>({greeting}) => {    var s = greeting.toLocaleLowerCase();  // Greeting should be of type string} : ({greeting}: { greeting: string; }) => void
+>greeting : string
+
+    var s = greeting.toLocaleLowerCase();  // Greeting should be of type string
+>s : string
+>greeting.toLocaleLowerCase() : string
+>greeting.toLocaleLowerCase : () => string
+>greeting : string
+>toLocaleLowerCase : () => string
+
+});
+
diff --git a/tests/cases/compiler/destructuringWithGenericParameter.ts b/tests/cases/compiler/destructuringWithGenericParameter.ts
@@ -0,0 +1,13 @@
+class GenericClass<T> {
+    payload: T;
+}
+
+var genericObject = new GenericClass<{ greeting: string }>();
+
+function genericFunction<T>(object: GenericClass<T>, callback: (payload: T) => void) {
+    callback(object.payload);
+}
+
+genericFunction(genericObject, ({greeting}) => {
+    var s = greeting.toLocaleLowerCase();  // Greeting should be of type string
+});
diff --git a/tests/cases/fourslash/parameterWithDestructuring.ts b/tests/cases/fourslash/parameterWithDestructuring.ts
@@ -0,0 +1,22 @@
+/// <reference path='fourslash.ts'/>
+
+// Repros from issues #4949 and #4818
+
+////const result = [{ foo: 'hello' }]
+////    .map(({ /*1*/foo }) => /*2*/foo)
+////    .map(foo => foo);
+////
+////const f = (foo: (bar: string[]) => void) => { };
+////
+////f(([a, b]) => {
+////    /*3*/a.charAt(0); // Not okay: inferred as `any`
+////});
+
+goTo.marker('1');
+verify.quickInfoIs('var foo: string');
+
+goTo.marker('2');
+verify.quickInfoIs('var foo: string');
+
+goTo.marker('3');
+verify.quickInfoIs('var a: string');