Improve typing for Generators and Async Generators

Author: rbuckton

src/compiler/checker.ts

@@ -39,6 +39,7 @@ namespace ts {
         ["es2017.string", "lib.es2017.string.d.ts"],
         ["es2017.intl", "lib.es2017.intl.d.ts"],
         ["es2017.typedarrays", "lib.es2017.typedarrays.d.ts"],
+        ["es2018.asyncgenerator", "lib.es2018.asyncgenerator.d.ts"],
         ["es2018.asynciterable", "lib.es2018.asynciterable.d.ts"],
         ["es2018.intl", "lib.es2018.intl.d.ts"],
         ["es2018.promise", "lib.es2018.promise.d.ts"],

src/compiler/commandLineParser.ts

@@ -3899,6 +3899,10 @@
         "category": "Message",
         "code": 6217
     },
+    "Enable strict checking of generator types.": {
+        "category": "Message",
+        "code": 6218
+    },
 
     "Projects to reference": {
         "category": "Message",
@@ -4256,6 +4260,10 @@
         "category": "Error",
         "code": 7051
     },
+    "Generator implicitly has type '{0}' because it does not yield or return any values. Consider supplying a return type.": {
+        "category": "Error",
+        "code": 7052
+    },
 
     "You cannot rename this element.": {
         "category": "Error",

src/compiler/diagnosticMessages.json

@@ -4226,13 +4226,21 @@ namespace ts {
         regularType: ResolvedType;  // Regular version of fresh type
     }
 
+    /* @internal */
+    export interface IterationTypes {
+        yieldType?: Type;
+        returnType?: Type;
+        nextType?: Type;
+    }
+
     // Just a place to cache element types of iterables and iterators
     /* @internal */
     export interface IterableOrIteratorType extends ObjectType, UnionType {
-        iteratedTypeOfIterable?: Type;
-        iteratedTypeOfIterator?: Type;
-        iteratedTypeOfAsyncIterable?: Type;
-        iteratedTypeOfAsyncIterator?: Type;
+        iterationTypesOfIterable?: IterationTypes;
+        iterationTypesOfIterator?: IterationTypes;
+        iterationTypesOfAsyncIterable?: IterationTypes;
+        iterationTypesOfAsyncIterator?: IterationTypes;
+        iterationTypesOfIteratorResult?: IterationTypes;
     }
 
     /* @internal */

src/compiler/types.ts

@@ -1,4 +1,11 @@
-interface Generator extends Iterator<any> { }
+/// <reference lib="es2015.iterable" />
+
+interface Generator<TYield = unknown, TReturn = void, TNext = unknown> {
+    next(value?: TNext): IteratorResult<TYield, TReturn>;
+    return(value: TReturn): IteratorResult<TYield, TReturn>;
+    throw(e: any): IteratorResult<TYield, TReturn>;
+    [Symbol.iterator](): Generator<TYield, TReturn, TNext>;
+}
 
 interface GeneratorFunction {
     /**