Report error for generators with return type void.

Bug: #32192
Change-Id: Ib31205207def253dc89802898a899b3555ecc28d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153953
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

Author: scheglov

pkg/analyzer/lib/src/error/return_type_verifier.dart

@@ -89,10 +89,24 @@ class ReturnTypeVerifier {
       ClassElement expectedElement,
       StaticTypeWarningCode errorCode,
     ) {
-      if (!_isLegalReturnType(expectedElement)) {
+      void reportError() {
         enclosingExecutable.hasLegalReturnType = false;
         _errorReporter.reportErrorForNode(errorCode, returnType);
       }
+
+      // It is a compile-time error if the declared return type of
+      // a function marked `sync*` or `async*` is `void`.
+      if (enclosingExecutable.isGenerator) {
+        if (enclosingExecutable.returnType.isVoid) {
+          return reportError();
+        }
+      }
+
+      // It is a compile-time error if the declared return type of
+      // a function marked `...` is not a supertype of `...`.
+      if (!_isLegalReturnType(expectedElement)) {
+        return reportError();
+      }
     }
 
     if (enclosingExecutable.isAsynchronous) {

pkg/analyzer/test/generated/static_type_warning_code_test.dart

@@ -380,6 +380,14 @@ SubStream<int> f() async* {}
     ]);
   }
 
+  test_illegalAsyncGeneratorReturnType_function_void() async {
+    await assertErrorsInCode('''
+void f() async* {}
+''', [
+      error(StaticTypeWarningCode.ILLEGAL_ASYNC_GENERATOR_RETURN_TYPE, 0, 4),
+    ]);
+  }
+
   test_illegalAsyncGeneratorReturnType_method_nonStream() async {
     await assertErrorsInCode('''
 class C {
@@ -402,6 +410,16 @@ class C {
     ]);
   }
 
+  test_illegalAsyncGeneratorReturnType_method_void() async {
+    await assertErrorsInCode('''
+class C {
+  void f() async* {}
+}
+''', [
+      error(StaticTypeWarningCode.ILLEGAL_ASYNC_GENERATOR_RETURN_TYPE, 12, 4),
+    ]);
+  }
+
   test_illegalSyncGeneratorReturnType_function_nonIterator() async {
     await assertErrorsInCode('''
 int f() sync* {}
@@ -419,6 +437,14 @@ SubIterator<int> f() sync* {}
     ]);
   }
 
+  test_illegalSyncGeneratorReturnType_function_void() async {
+    await assertErrorsInCode('''
+void f() sync* {}
+''', [
+      error(StaticTypeWarningCode.ILLEGAL_SYNC_GENERATOR_RETURN_TYPE, 0, 4),
+    ]);
+  }
+
   test_illegalSyncGeneratorReturnType_method_nonIterator() async {
     await assertErrorsInCode('''
 class C {
@@ -440,6 +466,16 @@ class C {
     ]);
   }
 
+  test_illegalSyncGeneratorReturnType_method_void() async {
+    await assertErrorsInCode('''
+class C {
+  void f() sync* {}
+}
+''', [
+      error(StaticTypeWarningCode.ILLEGAL_SYNC_GENERATOR_RETURN_TYPE, 12, 4),
+    ]);
+  }
+
   test_instanceAccessToStaticMember_method_reference() async {
     await assertErrorsInCode(r'''
 class A {

pkg/analyzer/test/src/diagnostics/illegal_async_return_type_test.dart

@@ -23,6 +23,12 @@ int f() async {}
     ]);
   }
 
+  test_function_nonFuture_void() async {
+    await assertNoErrorsInCode('''
+void f() async {}
+''');
+  }
+
   test_function_nonFuture_withReturn() async {
     await assertErrorsInCode('''
 int f() async {
@@ -55,6 +61,14 @@ class C {
     ]);
   }
 
+  test_method_nonFuture_void() async {
+    await assertNoErrorsInCode('''
+class C {
+  void m() async {}
+}
+''');
+  }
+
   test_method_subtypeOfFuture() async {
     await assertErrorsInCode('''
 import 'dart:async';