The "Cannot return value of type Foo on method returning type Bar" at the moment cannot be handled by typechecking extensions scripts.
It, alas, does happen with otherwise perfectly valid code like e.g.,
Of course, adding an explicit typecast into the source helps. But I've thought it's the typechecking extension very raison d'être to allow such things to go trough the typechecker without having to typecast explicitly in the code – the extension would say to the typechecker essentially "Yup, I know, it's all right here, let it pass".
The priority is minor since there are other possible work-arounds (namely, pusing the error collector and filtering these errors out later). Nevertheless direct support by a specific handler would be much better, for there would be more contextual information at the moment.