[ty] Add a diagnostic if a TypeVar is used to specialize a ParamSpec, or vice versa

[AlexWaygood]

Summary

Add validation to detect this error:

def f[**P](x: list[P]): ...

We already emitted an error for the reverse, i.e.

from typing import Callable

class Foo[**P, T]:
    x: Callable[P, T]

def f[T](x: Foo[T, T]): ...

but we didn't have any tests for it -- I added some.

Test Plan

mdtests and snapshots

[astral-sh-bot]

Typing conformance results improved 🎉

The percentage of diagnostics emitted that were expected errors increased from 85.73% to 85.74%. The percentage of expected errors that received a diagnostic increased from 78.45% to 78.54%.

Summary

Metric	Old	New	Diff	Outcome
True Positives	859	860	+1	⏫ (✅)
False Positives	143	143	+0
False Negatives	236	235	-1	⏬ (✅)
Total Diagnostics	1002	1003	+1	⏫
Precision	85.73%	85.74%	+0.01%	⏫ (✅)
Recall	78.45%	78.54%	+0.09%	⏫ (✅)

True positives added

Details

Location	Name	Message
generics_paramspec_basic.py:31:19	invalid-type-arguments	ParamSpec P cannot be used to specialize type variable _T

[astral-sh-bot]

Memory usage report

Memory usage unchanged ✅

[astral-sh-bot]

mypy_primer results

Changes were detected when running on open source projects

scikit-build-core (https://github.com/scikit-build/scikit-build-core)
+ src/scikit_build_core/build/wheel.py:99:20: error[no-matching-overload] No overload of bound method `__init__` matches arguments
- Found 57 diagnostics
+ Found 58 diagnostics