Fix nested Concatenate with ellipsis crash

[cyphercodes]

Fixes #21404.

This rejects a nested Concatenate[...] when it appears in the prefix arguments of another Concatenate, including the Concatenate[Concatenate[...], ...] crash case from the issue. The existing nested-Concatenate error path only handled the ParamSpec-prefix form, so the ellipsis form could reach Parameters(...) with another Parameters object nested inside it.

Tests:

• pytest -n0 mypy/test/testcheck.py::TypeCheckSuite::check-parameter-specification.test -k testStackedConcatenateIsIllegal -q
• pytest -n0 mypy/test/testcheck.py::TypeCheckSuite::check-parameter-specification.test -q
• python -m mypy --config-file mypy_self_check.ini mypy/typeanal.py
• python -m mypy --config-file mypy_self_check.ini -p mypy
• pre-commit run --files mypy/typeanal.py test-data/unit/check-parameter-specification.test
• git diff --check

[A5rocks]

IMO we should disallow Concatenate[...] there by construction, just like for Concatenate[P]. See:

from typing import Callable, Concatenate, ParamSpec

P = ParamSpec("P")
f: Callable[Concatenate[Concatenate[int, P], ...], object]  # E: Invalid location for Concatenate

Potentially related to #21425. I think this PR currently just duplicates effort.

[cyphercodes]

Updated this PR to address the latest review feedback from @A5rocks:

• Removed the local prefix-argument guard from apply_concatenate_operator.
• Treat Parameters returned by nested Concatenate[...] the same way as the existing invalid Concatenate location path in anal_type, so the new Concatenate[Concatenate[...], ...] case is rejected by construction with the standard diagnostic.

Local verification:

• python -m pytest -q 'mypy/test/testcheck.py::TypeCheckSuite::check-parameter-specification.test::testStackedConcatenateIsIllegal' ✅
• python -m pytest -q 'mypy/test/testcheck.py::TypeCheckSuite::check-parameter-specification.test::testParamSpecLocations' 'mypy/test/testcheck.py::TypeCheckSuite::check-parameter-specification.test::testParamSpecEllipsisInConcatenate' 'mypy/test/testcheck.py::TypeCheckSuite::check-parameter-specification.test::testStackedConcatenateIsIllegal' ✅
• git diff --check ✅

[cyphercodes]

Updated the expected output for the tuple/Concatenate data-case failure that showed up in CI.

Verification:

• python3 -m pytest mypy/test/testcheck.py::TypeCheckSuite::check-tuples.test::testTuplePassedParameters -q ✅
• git diff --check ✅

[A5rocks]

I think we can remove this old error codepath? I can't imagine a scenario where it's useful...

[A5rocks]

I think this can be simpler if you check isinstance for the 2 types up here. Then you can deduplicate the error codepaths.

[cyphercodes]

Updated this PR for the latest review feedback from @A5rocks:

• Simplified the allow_param_spec=False handling by checking ParamSpecType/Parameters together and sharing the final error-type assignment.
• Removed the old tuple[Concatenate[int, ...]] test case/error path from check-tuples.test.

Local verification:

• python3 -m pytest -q mypy/test/testcheck.py --tb=short -k 'testParamSpecLocations or testParamSpecEllipsisInConcatenate or testStackedConcatenateIsIllegal' ✅
• python3 -m pytest -q mypy/test/testcheck.py --tb=short -k 'check-tuples.test' ✅
• git diff --check ✅

[cyphercodes]

Updated this PR to fix the CI self-typecheck failure after the last review update.

Change:

• Added an explicit ParamSpecType narrowing assert before using .name in the non-Concatenate diagnostic path.

Local verification:

• python3 -m mypy --config-file mypy_self_check.ini -p mypy -p mypyc -n0 ✅
• python3 -m pytest -q mypy/test/testcheck.py --tb=short -k 'testParamSpecLocations or testParamSpecEllipsisInConcatenate or testStackedConcatenateIsIllegal' ✅
• python3 -m pytest -q mypy/test/testcheck.py --tb=short -k 'check-tuples.test' ✅
• git diff --check ✅

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅