PEP 696: Support earlier type param as default value

[hoodmane]

Bug Report

To Reproduce

type A[S, T=S] = dict[S, T]

a: A[int] = {}
reveal_type(a)

Expected Behavior

note: Revealed type is "builtins.dict[builtins.int, builtins.int]"

Actual Behavior

error: Name "T" is not defined  [name-defined]
note: Revealed type is "builtins.dict[builtins.int, Any]"

Your Environment

• Mypy version used: mypy 1.18.2 (compiled: yes)
• Mypy configuration options from mypy.ini (and other config files):

[tool.mypy]
python_version = "3.13"
show_error_codes = true

• Python version used: 3.13

I can't repro in the playground, instead it says:

error: Invalid syntax. Maybe you meant '==' or ':=' instead of '='?; you likely need to run mypy using Python 3.13 or newer  [syntax]

[sterliakov]

I think #14851 covers this? (adding pep695 because we do support defaulting to other typevars in "old-style" TypeVar definitions)

[sterliakov]

Oh, it's even better, your snippet already works on master - fixed in #20021! Playground errors because it runs python 3.12 even if you select a higher version, that's only --python-version flag value, apparently.

[hoodmane]

That's good to know. Would be great to have a release with this.