Do not generate a fake for a class which is only used as a nullable t…

…ype in a Future.

Fixes #409

Other containers do not feature this problem, like List, Stream, etc, as they are allowed to be empty. A Future can only be empty in this nullable type case.

PiperOrigin-RevId: 374242451

CHANGELOG.md

@@ -4,6 +4,8 @@
 * Support mocking methods with typed_data List return types.
 * Support mocking methods with return types declared in private SDK libraries
   (such as HttpClient and WebSocket, declared in `dart:_http`).
+* Do not generate a fake for a class which is only used as a nullable type in a
+  Future. [#409](https://github.com/dart-lang/mockito/issues/409)
 
 ## 5.0.7
 

lib/src/builder.dart

@@ -960,10 +960,14 @@ class _MockLibraryInfo {
     } else if (type.isDartCoreDouble) {
       return literalNum(0.0);
     } else if (type.isDartAsyncFuture || type.isDartAsyncFutureOr) {
-      var typeArgument = typeArguments.first;
+      final typeArgument = typeArguments.first;
+      final futureValueArguments =
+          typeSystem.isPotentiallyNonNullable(typeArgument)
+              ? [_dummyValue(typeArgument)]
+              : <Expression>[];
       return _futureReference(_typeReference(typeArgument))
           .property('value')
-          .call([_dummyValue(typeArgument)]);
+          .call(futureValueArguments);
     } else if (type.isDartCoreInt) {
       return literalNum(0);
     } else if (type.isDartCoreIterable) {

test/builder/auto_mocks_test.dart

@@ -526,7 +526,7 @@ void main() {
       '''),
       _containsAllOf(dedent2('''
       _i3.Future<void> m() => (super.noSuchMethod(Invocation.method(#m, []),
-          returnValue: Future<void>.value(null),
+          returnValue: Future<void>.value(),
           returnValueForMissingStub: Future.value()) as _i3.Future<void>);
       ''')),
     );
@@ -1836,6 +1836,22 @@ void main() {
     );
   });
 
+  test('creates dummy non-null return values for Futures of nullable types',
+      () async {
+    await expectSingleNonNullableOutput(
+      dedent('''
+      class Bar {}
+      class Foo {
+        Future<Bar?> m() async => null;
+      }
+      '''),
+      _containsAllOf(dedent2('''
+      _i3.Future<_i2.Bar?> m() => (super.noSuchMethod(Invocation.method(#m, []),
+          returnValue: Future<_i2.Bar?>.value()) as _i3.Future<_i2.Bar?>);
+      ''')),
+    );
+  });
+
   test(
       'creates dummy non-null return values for Futures of known typed_data classes',
       () async {