Type disappears from 3-member union in error message #7241

• Are you reporting a bug, or opening a feature request?

Bug

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

from typing import Callable, TypeVar, Union

T = TypeVar("T")

def f(
    get_value: Callable[[T], Union[bool, float, str]],
) -> Callable[[T], bool]:
    return lambda t: get_value(t) == 0

def g(get_value: Callable[[T], Union[None, bool, float, str]]):
    return f(get_value)

• What is the actual behavior/output?

a.py:11: error: Argument 1 to "f" has incompatible type "Callable[[T], Union[None, bool, float, str]]"; expected "Callable[[Any], Union[float, str]]"

• What is the behavior/output you expect?

a.py:11: error: Argument 1 to "f" has incompatible type "Callable[[T], Union[None, bool, float, str]]"; expected "Callable[[Any], Union[bool, float, str]]"

bool is missing from the actual output.

No such error if T is a concrete type, i.e. replace T = TypeVar("T") with class T: pass.

No such error if the union returned by get_value in f has only 2 members and the union returned by get_value in g has only 3 members.

• What are the versions of mypy and Python you are using?
  Do you see the same issue after installing mypy from Git master?

0.730+dev.9508e0efd2e89971f186440eac84eedbf991a4a9 (from master)

• What are the mypy flags you are using? (For example --strict-optional)

None