Union with TypeVar becomes generic

[ikonst]

With disallow_any_generics, this code:

from typing import TypeVar, Union

T1 = TypeVar('T1')
U1 = Union[T1, int]

x: U1 = 5

results in

main.py:6: error: Missing type parameters for generic type "U1"  [type-arg]

The U1 type alias becomes generic and has a single argument. Curiously, it doesn't matter what you put there. For example, this passes:

x: U1[str] = 5
#     ^^^

One might think it passes because T1 is unbound, but even if we bind it (e.g. to float), it still passes:

T1 = TypeVar('T1', bound=float)
#                        ^^^^^
U1 = Union[T1, int]

x: U1[str] = 5
#     ^^^

Playground:
https://mypy-play.net/?mypy=latest&python=3.11&flags=disallow-any-generics&gist=68115745200d34208786d8267d2d324a

[AlexWaygood]

With disallow_any_generics, this code:

from typing import TypeVar, Union

T1 = TypeVar('T1')
U1 = Union[T1, int]

x: U1 = 5

results in

main.py:6: error: Missing type parameters for generic type "U1"  [type-arg]

The U1 type alias becomes generic and has a single argument.

This is a feature, not a bug. You've created a generic type alias. The feature is documented here: https://mypy.readthedocs.io/en/stable/generics.html#generic-type-aliases

Curiously, it doesn't matter what you put there. For example, this passes:

x: U1[str] = 5
#     ^^^

This doesn't look like a bug to me. U1[str] evaluates to Union[str, int], and it's okay to assign the value 5 to an object that has the type Union[str, int], since 5 is an int.

One might think it passes because T1 is unbound, but even if we bind it (e.g. to float), it still passes:

T1 = TypeVar('T1', bound=float)
#                        ^^^^^
U1 = Union[T1, int]

x: U1[str] = 5
#     ^^^

This one is a bug. However, it's a duplicate of #10445 🙂. It's already been fixed on mypy master; the fix will be included as part of mypy 1.0 (the next release).

If you try the following snippet on mypy master, here is what mypy reports:

from typing import TypeVar, Union

T1 = TypeVar('T1', bound=float)
U1 = Union[T1, int]

x: U1[str] = 5  # error: Type argument "str" of "U1" must be a subtype of "float" [type-var]

[ikonst]

Thanks for the thorough answer.