type variable escapes in application of higher-order function to a generic function

[rwbarton]

from typing import TypeVar, Callable
S = TypeVar('S')
def apply(f: Callable[[int], S]) -> S: pass
def g(x: int) -> S: pass
y = apply(g)
reveal_type(y) # E: Revealed type is 'S`-1'

The type of y is not allowed to contain an unbound type variable. There should have been an error Need type annotation for variable on the assignment to y.

(With apply having type def apply(f: Callable[[int], int]) -> S, mypy does give such an error.)

[rwbarton]

Also, even when the input program has a type error, mypy should still not produce ill-formed types like S-1` here. That way we can run mypy in a mode with extra consistency checks on the test suite, which is full of intentional type errors, to find more bugs like this one.

[erictraut]

Mypy is arguably doing the right thing in this case. The code sample contains a malformed generic function g that returns a generic S but doesn't use S in the types of any input parameters. Mypy correctly reports this error.