Can't stub generic function with nested function param of unknown type

[AlexandrKedrov]

Code:

abstract class Foo {
    T? foo<T>(Map<String, T Function()> options);
}

T? fooShim<T>(Map<String, T Function()>? options> {
    return null;
}

@GenerateMocks([], customMocks[
    MockSpec<Foo>(fallbackGenerators: {#foo: fooShim})
])
...

Error:

Invalid @GenerateMocks annotation: Mockito cannot generate a valid mock class which implements 'Foo' for the following reasons:
    The method 'Foo.foo' features a non-nullable unknown return type, and cannot be stubbed without a dummy generator specified on the MockSpec.

Call stack:

Problem is that neither hasFallbackGenerator nor isParameter passed to the second _checkFunction call.

As stubbing generic function with Map<> or T Function() should work(at least I hope so according to the current implementation), it is ambiguous their composition causes any trouble

[srawlins]

Thanks for the bug report and the initial investigation!