Not evaluating Union[X, Y] from Type[Union[X, Y]] over (Type[T]) -> T function

[ericbn]

Bug Report

Mypy is evaluating builtins.object instead of Union[X, Y] as the returned type when passing an argument of type Type[Union[X, Y]] to a def [T] (cl: Type[T']) -> T' function.

To Reproduce

This works as expected:

from typing import TypeVar

T = TypeVar("T")


def _structure(cl: type[T]) -> T:
    return cl()


def structure(cl: type[int]) -> int:
    reveal_type(cl)
    reveal_type(_structure)
    data = _structure(cl)
    reveal_type(data)
    return data

Output:

scratch.py:11: note: Revealed type is "Type[builtins.int]"
scratch.py:12: note: Revealed type is "def [T] (cl: Type[T`-1]) -> T`-1"
scratch.py:14: note: Revealed type is "builtins.int"
Success: no issues found in 1 source file

Using str instead of int above also works as expected the same way.

But this does not:

from typing import TypeVar

T = TypeVar("T")


def _structure(cl: type[T]) -> T:
    return cl()


def structure(cl: type[int | str]) -> int | str:
    reveal_type(cl)
    reveal_type(_structure)
    data = _structure(cl)
    reveal_type(data)
    return data

Expected Behavior

Output:

scratch.py:11: note: Revealed type is "Union[Type[builtins.int], Type[builtins.str]]"
scratch.py:12: note: Revealed type is "def [T] (cl: Type[T`-1]) -> T`-1"
scratch.py:14: note: Revealed type is "Union[builtins.int, builtins.str]"
Success: no issues found in 1 source file

or

scratch.py:11: note: Revealed type is "Type[Union[builtins.int, builtins.str]]"
scratch.py:12: note: Revealed type is "def [T] (cl: Type[T`-1]) -> T`-1"
scratch.py:14: note: Revealed type is "Union[builtins.int, builtins.str]"
Success: no issues found in 1 source file

(Type[Union[X, Y]] instead of Union[Type[X], Type[Y]] in line 11)

Actual Behavior

Output:

scratch.py:11: note: Revealed type is "Union[Type[builtins.int], Type[builtins.str]]"
scratch.py:12: note: Revealed type is "def [T] (cl: Type[T`-1]) -> T`-1"
scratch.py:14: note: Revealed type is "builtins.object"
scratch.py:15: error: Incompatible return value type (got "object", expected "int | str")  [return-value]
Found 1 error in 1 file (checked 1 source file)

(revealed type in line 14 is "builtins.object" instead of "Union[builtins.int, builtins.str]")

The error looks like might be because of the covariance of Union.

Your Environment

• Mypy version used: 1.4.1
• Mypy command-line flags: just the file name, no extra flags.
• Mypy configuration options from mypy.ini (and other config files):

  [tool.mypy]
  platform = "linux"
  python_version = "3.10"
  

• Python version used: 3.10.12
• Operating system and version: macOS 12.6.7

[ericbn]

Updated the issue description. It was initially reported using mypy version 0.971 and it's still present in mypy 1.4.1.