Check return type inside generators

src/compiler/checker.ts

@@ -23219,14 +23219,15 @@ namespace ts {
             const returnType = getReturnTypeOfSignature(signature);
             const functionFlags = getFunctionFlags(func);
             const isGenerator = functionFlags & FunctionFlags.Generator;
+            const isAsync = (functionFlags & FunctionFlags.Async) !== 0;
             if (strictNullChecks || node.expression || returnType.flags & TypeFlags.Never) {
                 const exprType = node.expression ? checkExpressionCached(node.expression) : undefinedType;
-                if (isGenerator) { // AsyncGenerator function or Generator function
-                    // A generator does not need its return expressions checked against its return type.
-                    // Instead, the yield expressions are checked against the element type.
-                    // TODO: Check return types of generators when return type tracking is added
-                    // for generators.
-                    return;
+                if (isGenerator) {
+                    const returnType = getEffectiveReturnTypeNode(func);
+                    if (returnType) {
+                        const signatureElementType = getIteratedTypeOfGenerator(getTypeFromTypeNode(returnType), isAsync) || anyType;
+                        checkTypeAssignableTo(exprType, signatureElementType, node);
+                    }
                 }
                 else if (func.kind === SyntaxKind.SetAccessor) {
                     if (node.expression) {

tests/baselines/reference/generatorTypeCheck12.errors.txt

@@ -0,0 +1,9 @@
+tests/cases/conformance/es6/yieldExpressions/generatorTypeCheck12.ts(2,5): error TS2322: Type '""' is not assignable to type 'number'.
+
+
+==== tests/cases/conformance/es6/yieldExpressions/generatorTypeCheck12.ts (1 errors) ====
+    function* g(): IterableIterator<number> {
+        return "";
+        ~~~~~~~~~~
+!!! error TS2322: Type '""' is not assignable to type 'number'.
+    }

tests/baselines/reference/generatorTypeCheck13.errors.txt

@@ -0,0 +1,10 @@
+tests/cases/conformance/es6/yieldExpressions/generatorTypeCheck13.ts(3,5): error TS2322: Type '""' is not assignable to type 'number'.
+
+
+==== tests/cases/conformance/es6/yieldExpressions/generatorTypeCheck13.ts (1 errors) ====
+    function* g(): IterableIterator<number> {
+        yield 0;
+        return "";
+        ~~~~~~~~~~
+!!! error TS2322: Type '""' is not assignable to type 'number'.
+    }