Error processing type parameters in stubs for Python <3.12

[Andrej730]

Bug Report

mypy shows a syntax error ("Type parameter lists are only supported in Python 3.12 and greater") meeting type parameters in stub files.

To Reproduce

test.pyi

class A[T]:
    def __init__(self, a: T):
        self.a = a


def test(a: A[int]):
    a.a

And run 3 following commands:

py -3.11 -m mypy test.pyi
py -3.12 -m mypy test.pyi
py -3.12 -m mypy test.pyi --python-version 3.9

Expected Behavior

For mypy finish without errors in all cases since docs state that stub files support all latest typing features except type keyword.

https://typing.readthedocs.io/en/latest/guides/writing_stubs.html?utm_source=chatgpt.com#language-features

Actual Behavior

Success only for Python 3.12+ with python-version 3.12+.

> py -3.11 -m mypy test.pyi
test.pyi:2: error: invalid syntax  [syntax]
Found 1 error in 1 file (errors prevented further checking)

> py -3.12 -m mypy test.pyi
Success: no issues found in 1 source file

> py -3.12 -m mypy test.pyi --python-version 3.9
test.pyi:2: error: Type parameter lists are only supported in Python 3.12 and greater  [syntax]
Found 1 error in 1 file (errors prevented further checking)

Your Environment

• Mypy version used: 1.15
• Python version used: 3.11 / 3.12

[hauntsaninja]

This is the expected behaviour. The type keyword in the docs you link is just an example.

On Python 3.11, there is nothing mypy can do, since it relies on the ast module to parse code, and Python 3.11 ast cannot parse code using Python 3.12 features.
On Python 3.12, if you explicitly specify a Python version, mypy will type check against that Python version. This is a feature and is relied on by folks to ensure their code is backwards compatible if type checked in environments with older Python