typevars becoming types with typevars doesn't work

[ddfisher]

from typing import TypeVar
T = TypeVar('T')
V = TypeVar('V')

def identity(x: T) -> T: ...
def type_var_using_fn(x: V) -> None: ...
identity(type_var_using_fn)

has the confusing error:

test.py:10: error: Argument 1 to "identity" has incompatible type Callable[[V], None]; expected Callable[[V], None]

This is most commonly seen when decorating functions with TypeVars.

[JukkaL]

This could be another manifestation of the old type variable renaming bug (#230 or #603 may be related.)

[rwbarton]

Here's what's happening here:

• We need to typecheck the expression identity(type_var_using_fn).
• Since identity is a generic function, we have to infer the correct T to pick for the call based on the argument. mypy correctly determines that T = def [V] (x: V-1), the type of the generic function type_var_using_fn`.
• Having chosen T, we need to check that the arguments are actually valid. In this case, this means checking that the actual argument type def [V] (x: V-1)is a subtype of the formal argument typeT = def [V] (x: V-1). Of course it is, but checking whether a callable is a subtype of a generic callable is not yet supported by mypy, resulting in the confusing error message.

I think the mypythonic thing to do here is to just implement this subtyping check: A callable is a subtype of a generic callable if it is so after the generic callable is instantiated at fresh ("skolem") types.

[ddfisher]

Thanks for tracking this down! I agree, implementing that subtyping check sounds like the right fix here.

[gvanrossum]

Thanks! That sounds like a great analysis. Can you pull of the fix without more help?

[rwbarton]

I have what should be a fix, but encountered #1335 while testing it.