`TypeGuard` doesn't work with `TypeVar`

[MicaelJarniac]

Bug Report

Using a TypeVar with TypeGuard results in Mypy thinking that the resulting type is the TypeVar itself, and not the type it should be pointing to.

To Reproduce

Example adapted from PEP 647:

# src/test.py

from typing import TypeGuard, TypeVar

_T = TypeVar("_T")


def is_two_element_tuple(val: tuple[_T, ...]) -> TypeGuard[tuple[_T, _T]]:
    return len(val) == 2


def func(names: tuple[str, ...]):
    if is_two_element_tuple(names):
        reveal_type(names)  # Tuple[str, str]
        for name in names:
            print(name.capitalize(), "- two")
    else:
        reveal_type(names)  # Tuple[str, ...]
        for name in names:
            print(name.capitalize(), "- not two")

Expected Behavior

Mypy should substitute the TypeVar inside TypeGuard to the proper type it should be pointing to.
Pyre, Pyright, and Pylance do successfully work with the TypeVar inside TypeGuard.

$ pyre
ƛ No type errors found
src/test.py:14:8 Revealed type [-1]: Revealed type for `names` is `typing.Tuple[str, str]`.
src/test.py:18:8 Revealed type [-1]: Revealed type for `names` is `typing.Tuple[str, ...]`.

$ pyright src/test.py
/home/micael/projects/python-3_10/src/test.py
  /home/micael/projects/python-3_10/src/test.py:14:21 - info: Type of "names" is "tuple[str, str]"
  /home/micael/projects/python-3_10/src/test.py:18:21 - info: Type of "names" is "tuple[str, ...]"
0 errors, 0 warnings, 2 infos

Actual Behavior

$ mypy src/test.py
src/test.py:14: note: Revealed type is "Tuple[_T`-1, _T`-1]"
src/test.py:16: error: "_T" has no attribute "capitalize"
src/test.py:18: note: Revealed type is "builtins.tuple[builtins.str]"
src/test.py:19: error: Incompatible types in assignment (expression has type "str", variable has type "_T")
src/test.py:20: error: "_T" has no attribute "capitalize"
Found 3 errors in 1 file (checked 1 source file)

Your Environment

Name	Version
Python	3.10.0
typing_extensions	4.0.1
mypy	0.920
mypy-extensions	0.4.3
pyre-check	0.9.8
pyre-extensions	0.0.23
pyright	0.0.13

• Mypy version used: 0.920
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10.0
• Operating system and version: Manjaro KDE

Operating System: Manjaro Linux
KDE Plasma Version: 5.23.4
KDE Frameworks Version: 5.88.0
Qt Version: 5.15.2
Kernel Version: 5.15.7-1-MANJARO (64-bit)
Graphics Platform: X11
Processors: 4 × AMD Ryzen 3 2200G with Radeon Vega Graphics
Memory: 7.7 GiB of RAM
Graphics Processor: NVIDIA GeForce GTX 970/PCIe/SSE2

[wbolster]

i think this can be closed as a duplicate of #11428, @MicaelJarniac? (as you noticed yourself)

[MicaelJarniac]

I believe the bug in question is to do with TypeVar, and not Generic, as your issue implies.

And in my opinion, the examples I've given here are easier to understand and more straight to the point.

I'm not sure about what to do.

One thing I did not test is whether the issue I'm describing here also happens on Python 3.10, and that's why this issue is about the TypeGuard from typing_extensions.
But the issue you're having seems to be happening on at least Python 3.10.

I do think our issues are about the same bug, but we've come across it in different ways.

I'll try to test my examples against Python 3.10, to see whether it's in any way related to typing_extensions or not.
If it is still happening on 3.10, then I do think we could look into merging both issues onto one.

[MicaelJarniac]

Just tested it with Python 3.10.0 and using the built-in TypeGuard from typing, and the bug is still there.

[JelleZijlstra]

Closing as a duplicate of #11428.