mypy shows a somewhat confusing error message for functions returning a TypeVar value

Not sure if this should be considered a bug, or an enhancement proposal...

I had some code looking like this:

CallbackResult = TypeVar("CallbackResult")
VisitCallback = Callable[[str], Optional[CallbackResult]]

def visititems(self, func: VisitCallback) -> CallbackResult:
    return func("test")

Actual Behavior

mypy says about this code:

error: A function returning TypeVar should receive at least one argument containing the same TypeVar  [type-var]

Expected Behavior

First, I was confused, and thought that mypy is not able to infer that CallbackResult is hidden in the function input as the return value of VisitCallback. Actually I was initially going to open an issue because of that.

Then I noticed that in a sense, mypy is correct - because I spotted that VisitCallback returns Optional[CallbackResult], while the visititems function returns a CallbackResult.

Once I fixed that, the error went away. But in this case, mypy did not really "pinpoint" the problem as well as I feel it could.

Not sure how deeply mypy plays out the type algebra to figure out what types are constructible in a context, but I feel like this specific case could be a rather "common" one (forgetting the Optional), so maybe this can be catched with a better error message.

Maybe something of the form "the function returns typevar value X, but based on its inputs can only construct Y[X, ...]" could be possible? So that it would cover Optional, Union and other simple cases like List, etc.

Your Environment

• Mypy version used: 1.1.1

Comienza reproduciendo el ejemplo de TypeVar del issue y examinando cómo mypy informa del error de type-var cuando se devuelve Optional[CallbackResult] como CallbackResult. Se considera terminado cuando se produzca un diagnóstico más claro que explique la discrepancia y cubra wrappers simples como Optional, Union o List.