Move Iterable type transformation function into its own class for r…

…euse.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=228000581

src/com/google/javascript/jscomp/Es6RewriteGenerators.java

@@ -266,7 +266,7 @@ private class SingleGeneratorFunctionTranspiler {
         if (genFunc.getJSType() != null && genFunc.getJSType().isFunctionType()) {
           FunctionType fnType = genFunc.getJSType().toMaybeFunctionType();
           this.originalGenReturnType = fnType.getReturnType();
-          yieldType = TypeCheck.getTemplateTypeOfGenerator(registry, originalGenReturnType);
+          yieldType = JsIterables.getElementType(originalGenReturnType, registry);
         }
 
         JSType globalContextType = registry.getGlobalType("$jscomp.generator.Context");

src/com/google/javascript/jscomp/JsIterables.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2014 The Closure Compiler Authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.javascript.jscomp;
+
+import static com.google.javascript.rhino.jstype.JSTypeNative.UNKNOWN_TYPE;
+
+import com.google.javascript.rhino.jstype.JSType;
+import com.google.javascript.rhino.jstype.JSTypeRegistry;
+import com.google.javascript.rhino.jstype.TemplateTypeMap;
+
+/**
+ * Models type transformations of JavaScript `Iterable` and `Iterator` types.
+ *
+ * <p>These tranformations can be especially helpful when working with generator functions.
+ * `Generator` is a subtype of `Iterable`.
+ */
+final class JsIterables {
+
+  /**
+   * Returns the given `Iterable`s element type.
+   *
+   * <p>If the given type is not an `Iterator` or `Iterable`, returns the unknown type.
+   */
+  static final JSType getElementType(JSType iterableOrIterator, JSTypeRegistry typeRegistry) {
+    TemplateTypeMap templateTypeMap =
+        iterableOrIterator
+            // Remember that `string` will box to a `Iterable`.
+            .autobox()
+            .getTemplateTypeMap();
+
+    if (templateTypeMap.hasTemplateKey(typeRegistry.getIterableTemplate())) {
+      // `Iterable<SomeElementType>` or `Generator<SomeElementType>`
+      return templateTypeMap.getResolvedTemplateType(typeRegistry.getIterableTemplate());
+    } else if (templateTypeMap.hasTemplateKey(typeRegistry.getIteratorTemplate())) {
+      // `Iterator<SomeElementType>`
+      return templateTypeMap.getResolvedTemplateType(typeRegistry.getIteratorTemplate());
+    }
+    return typeRegistry.getNativeType(UNKNOWN_TYPE);
+  }
+
+  private JsIterables() {}
+}

src/com/google/javascript/jscomp/TypeCheck.java

@@ -2552,7 +2552,7 @@ private void visitReturn(NodeTraversal t, Node n) {
       } else if (enclosingFunction.isGeneratorFunction()) {
         // Unwrap the template variable from a generator function's declared return type.
         // e.g. if returnType is "Generator<string>", make it just "string".
-        returnType = getTemplateTypeOfGenerator(returnType);
+        returnType = JsIterables.getElementType(returnType, typeRegistry);
       } else if (enclosingFunction.isAsyncFunction()) {
         // e.g. `!Promise<string>` => `string|!IThenable<string>`
         // We transform the expected return type rather than the actual return type so that the
@@ -2592,7 +2592,7 @@ private void visitYield(NodeTraversal t, Node n) {
     if (jsType.isFunctionType()) {
       FunctionType functionType = jsType.toMaybeFunctionType();
       JSType returnType = functionType.getReturnType();
-      declaredYieldType = getTemplateTypeOfGenerator(returnType);
+      declaredYieldType = JsIterables.getElementType(returnType, typeRegistry);
     }
 
     // fetching the yielded value's type
@@ -2624,32 +2624,6 @@ private void visitYield(NodeTraversal t, Node n) {
         "Yielded type does not match declared return type.");
   }
 
-  private JSType getTemplateTypeOfGenerator(JSType generator) {
-    return getTemplateTypeOfGenerator(typeRegistry, generator);
-  }
-
-  /**
-   * Returns the given type's resolved template type corresponding to the corresponding to the
-   * Generator, Iterable or Iterator template key if possible.
-   *
-   * <p>If the given type is not an Iterator or Iterable, returns the unknown type..
-   */
-  static JSType getTemplateTypeOfGenerator(JSTypeRegistry typeRegistry, JSType generator) {
-    TemplateTypeMap templateTypeMap = generator.autobox().getTemplateTypeMap();
-    if (templateTypeMap.hasTemplateKey(typeRegistry.getIterableTemplate())) {
-      // Generator JSDoc says
-      // @return {!Iterable<SomeElementType>}
-      // or
-      // @return {!Generator<SomeElementType>}
-      return templateTypeMap.getResolvedTemplateType(typeRegistry.getIterableTemplate());
-    } else if (templateTypeMap.hasTemplateKey(typeRegistry.getIteratorTemplate())) {
-      // Generator JSDoc says
-      // @return {!Iterator<SomeElementType>}
-      return templateTypeMap.getResolvedTemplateType(typeRegistry.getIteratorTemplate());
-    }
-    return typeRegistry.getNativeType(UNKNOWN_TYPE);
-  }
-
   private void visitTaggedTemplateLit(NodeTraversal t, Node n) {
     Node tag = n.getFirstChild();
     JSType tagType = tag.getJSType().restrictByNotNullOrUndefined();