Optimize direct Iterable reference in getIteratedType

src/compiler/checker.ts

@@ -9346,6 +9346,12 @@ module ts {
                     return undefined;
                 }
 
+                // As an optimization, if the type is instantiated directly using the globalIterableType (Iterable<number>),
+                // then just grab its type argument.
+                if ((iterable.flags & TypeFlags.Reference) && (<GenericType>iterable).target === globalIterableType) {
+                    return (<GenericType>iterable).typeArguments[0];
+                }
+
                 let iteratorFunction = getTypeOfPropertyOfType(iterable, getPropertyNameForKnownSymbolName("iterator"));
                 if (iteratorFunction && allConstituentTypesHaveKind(iteratorFunction, TypeFlags.Any)) {
                     return undefined;

tests/baselines/reference/for-of57.js

@@ -0,0 +1,8 @@
+//// [for-of57.ts]
+var iter: Iterable<number>;
+for (let num of iter) { }
+
+//// [for-of57.js]
+var iter;
+for (let num of iter) {
+}

tests/baselines/reference/for-of57.types

@@ -0,0 +1,9 @@
+=== tests/cases/conformance/es6/for-ofStatements/for-of57.ts ===
+var iter: Iterable<number>;
+>iter : Iterable<number>
+>Iterable : Iterable<T>
+
+for (let num of iter) { }
+>num : number
+>iter : Iterable<number>
+

tests/baselines/reference/iteratorSpreadInArray11.js

@@ -0,0 +1,9 @@
+//// [iteratorSpreadInArray11.ts]
+var iter: Iterable<number>;
+var array = [...iter];
+
+//// [iteratorSpreadInArray11.js]
+var iter;
+var array = [
+    ...iter
+];

tests/baselines/reference/iteratorSpreadInArray11.types

@@ -0,0 +1,11 @@
+=== tests/cases/conformance/es6/spread/iteratorSpreadInArray11.ts ===
+var iter: Iterable<number>;
+>iter : Iterable<number>
+>Iterable : Iterable<T>
+
+var array = [...iter];
+>array : number[]
+>[...iter] : number[]
+>...iter : number
+>iter : Iterable<number>
+

tests/cases/conformance/es6/for-ofStatements/for-of57.ts

@@ -0,0 +1,3 @@
+//@target: ES6
+var iter: Iterable<number>;
+for (let num of iter) { }

tests/cases/conformance/es6/spread/iteratorSpreadInArray11.ts

@@ -0,0 +1,3 @@
+//@target: ES6
+var iter: Iterable<number>;
+var array = [...iter];