TypeAliasType is always generic when type_params=() is passed.

[Daraan]

Bug report

Bug description:

I realized that the type_params parameter of TypeAliasType behaves differently when it is omitted vs passing an empty tuple (), in the latter case the resulting instance is always subscriptable. I doubt this is an undocumented feature.

from typing import List, TypeAliasType, TypeVar

# < Errors like expected >

Simple = TypeAliasType("Simple", int)
try:
    Simple[int]
except TypeError:
   print("Expected: Simple is not subcriptable")

# Not allowed assignment
T = TypeVar('T')
MissingTypeParamsErr = TypeAliasType("MissingTypeParamsErr", List[T])
assert MissingTypeParamsErr.__type_params__ == ()
assert MissingTypeParamsErr.__parameters__ == ()

try:
    MissingTypeParamsErr[int]
except TypeError:
   print("Expected: Simple is not subcriptable")

# < unexpected cases >
    
MissingTypeParams = TypeAliasType("MissingTypeParams", List[T], type_params=())
assert MissingTypeParams.__type_params__ == ()
assert MissingTypeParams.__parameters__ == ()
# These cases do not raise an error
MissingTypeParams[int]
MissingTypeParams[[]]
MissingTypeParams[()]

# This also do not raise an error
Simple2 = TypeAliasType("Simple2", int, type_params=())
assert Simple2 .__type_params__ == ()
assert Simple2 .__parameters__ == ()
Simple2[int]
Simple2[[]]
Simple2[()]

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-124498: Fix TypeAliasType not to be generic, when type_params=() #124499
• [3.13] gh-124498: Fix TypeAliasType not to be generic, when type_params=() (GH-124499) #124603
• [3.12] gh-124498: Fix TypeAliasType not to be generic, when type_params=() (GH-124499) #124604