a second TypeVar confuses mypy in @overload return types

[finite-state-machine]

Bug Report

When an overloaded function implementation returns a Union, mypy becomes confused when that Union contains two TypeVars.

The following code works as expected (i.e., there are no errors):

from typing import (
        Generic,
        Literal,
        Optional,
        overload,
        TypeVar,
        Union,
        )

T1 = TypeVar('T1')
T2 = TypeVar('T2')

# first example, works as expected:

U1 = Union[None, T1]

@overload
def works1(number: Literal[1]) -> T1: ...
@overload
def works1(number: Literal[2]) -> Optional[T1]: ...
def works1(number: int) -> U1:
    pass

# second example, also works as expected:

U2 = Union[None, int, T2]

@overload
def works2(number: Literal[1]) -> int: ...
@overload
def works2(number: Literal[2]) -> Optional[int]: ...
def works2(number: int) -> U2:
    pass

Adding T2 to U1 in the first example, or changing int to T1 in the second example, produces this third example, which exhibits the problem in question:

from typing import (
        Generic,
        Literal,
        Optional,
        overload,
        TypeVar,
        Union,
        )

T1 = TypeVar('T1')
T2 = TypeVar('T2')

# third example, does not work as expected:

U3 = Union[None, T1, T2]

@overload
def function(number: Literal[1]) -> T1: ...
@overload
def function(number: Literal[2]) -> Optional[T1]: ...
def function(number: int) -> U3:
    pass
# error: Overloaded function implementation cannot produce return type of signature 1  [misc]
# error: Overloaded function implementation cannot produce return type of signature 2  [misc]

To Reproduce

Run the "third example" code through mypy.

Expected Behavior

There should be no errors when processing the third example.

Actual Behavior

These errors are produced:

# error: Overloaded function implementation cannot produce return type of signature 1  [misc]
# error: Overloaded function implementation cannot produce return type of signature 2  [misc]

Further information

This may be related to #9023.

It may be helpful to know that the same problem still occurs if function is a method of a type which inherits from Generic[T2] (i.e., binding T2 doesn't help):

class SomeClass(Generic[T2]):
    @overload
    def method(self, number: Literal[1]) -> T1: ...
    @overload
    def method(self, number: Literal[2]) -> Optional[T1]: ...
    def method(self, number: int) -> Union[None, T1, T2]:
        pass

Your Environment

• Mypy version used: 0.910 (installed via pip)
• Mypy command-line flags: (none necessary to reproduce the issue)
• Mypy configuration options from mypy.ini (and other config files): (none necessary to reproduce the issue)
• Python version used: 3.8.3 as installed by pyenv
• Operating system and version: macOS 10.14.6

[A5rocks]

This works on mypy master, by the way.

[hauntsaninja]

I can't reproduce on any recent version of mypy