Fix native parser for quoted None types

[daleselaji-dev]

Fixes #22090

Problem

With the native parser enabled, cast("None", value) is rejected with Invalid type comment or annotation, although the same quoted forward reference is accepted by the legacy parser and was accepted before the ast-serialize 0.12.0 upgrade.

Root Cause

The native type-string parser represents the literal None as a RawExpressionType instead of the UnboundType("None") produced by the legacy parser. Semantic analysis therefore treats the cast target as an invalid string expression.

Solution

Normalize the native parser's exact None result to UnboundType("None"), preserving the existing forward-reference behavior without changing other raw expression handling.

Changes

• Normalize the native parser result for the exact None type string.
• Add a regression case covering cast("None", value).

Testing

• Baseline: check-newsemanal.test — 226 passed.
• Focused regression: check-newsemanal.test — 227 passed.
• Native parser suite: mypy/test/test_nativeparse.py — 257 passed.
• Native and legacy CLI repros: both report success for cast("None", value).
• pre-commit run --files mypy/nativeparse.py test-data/unit/check-newsemanal.test — passed (Black, Ruff, codespell and repository hooks).
• python -m mypy --config-file mypy_self_check.ini -p mypy — 196 source files passed.
• git diff --check — passed.
• Full test suite and multi-platform CI matrix — not run locally.

Compatibility/Risk

The change only affects the native parser's handling of the exact quoted None type expression. Other raw expression types and the legacy parser are unchanged.

Notes for Reviewer

The regression is exposed by the ast-serialize 0.12.0 parser output. The fix keeps the compatibility behavior already provided by the legacy parser and is limited to the parser boundary.

AI assistance was used during investigation and implementation; the final diff and test results are documented above.

Linked Issue

Fixes #22090

[github-actions]

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