Dart currently lets us write

T someFunc<T extends Object>(T Function() f) {...}

I repeatedly make the mistake of passing Future<T> Function() to a function that expects T Function(). These mistakes often cause runtime errors. I wish to prevent such errors with a type constraint that means "T does not extend Future". Proposed syntax:

T someFunc<T extends Object + !Future>(T Function() f) {...}

A linter warning would also solve the problem.