[DipanshuAmbilkar03]

Fixes #22202

Summary

match False: is not treated as exhaustive under case True / case False, and a following case other is reachable even though the subject can only be False:

def f() -> int:
    match False:
        case True:
            return 1
        case False:
            return 0

def h() -> int:
    match False:
        case True:
            return 1
        case False:
            return 0
        case other:
            reveal_type(other)  # Literal[False]
            return 2

The subject expression False has type bool with a last_known_value of Literal[False]. To narrow against a bool literal pattern, conditional_types() expands the bool instance into Literal[True] | Literal[False], but that expansion discarded the known value. Matching against case False therefore left Literal[True] as the remaining type, so the match was not exhaustive and case other stayed reachable.

Fix

I changed try_expanding_sum_type_to_union() so that an instance carrying a last_known_value expands to that literal instead of the full union. Plain bool and enum instances have no last_known_value, so their behaviour is unchanged.

Test

testMatchBoolLiteralSubjectIsExhaustive in test-data/unit/check-python310.test covers:

• match False with case True / case False being exhaustive (no missing return);
• the unreachable case True bodies being reported under --warn-unreachable;
• a case other after them being unreachable;
• the same match on a plain bool parameter still behaving as before.

Verification

• pytest mypy/test/testcheck.py -k Match — 262 passed, 1 skipped.
• pytest mypy/test/testcheck.py — 8341 passed, 27 skipped, 7 xfailed.
• pytest mypy/test (excluding testmypyc.py, which needs a mypyc build) — 12557 passed, 362 skipped, 10 xfailed.
• python -m mypy --config-file mypy_self_check.ini -p mypy — clean.

[github-actions]

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