Narrow unreachable captures due to guards in match to Never

Author: A5rocks

mypy/checker.py

@@ -5515,6 +5515,9 @@ def visit_match_stmt(self, s: MatchStmt) -> None:
                             if isinstance(p, AsPattern):
                                 case_target = p.pattern or p.name
                                 if isinstance(case_target, NameExpr):
+                                    if guard_map is None:
+                                        guard_map = {case_target: UninhabitedType()}
+
                                     for type_map in (guard_map, else_map):
                                         if not type_map:
                                             continue

test-data/unit/check-narrowing.test

@@ -995,7 +995,6 @@ else:
     reveal_type(a)  # E: Statement is unreachable \
                     # N: Revealed type is "Never"
     reveal_type(b)  # N: Revealed type is "Union[Literal[1], Literal[2], Literal[3], Literal[4]]"
-
 [builtins fixtures/primitives.pyi]
 
 [case testNarrowingLiteralTruthiness]
@@ -2112,8 +2111,6 @@ if isinstance(x, (Z, NoneType)):  # E: Subclass of "X" and "Z" cannot exist: "Z"
                                   # E: Subclass of "X" and "NoneType" cannot exist: "NoneType" is final
     reveal_type(x)  # E: Statement is unreachable \
                     # N: Revealed type is "Never"
-
-
 [builtins fixtures/isinstance.pyi]
 
 [case testTypeNarrowingReachableNegative]

test-data/unit/check-python310.test

@@ -1329,7 +1329,7 @@ m: str
 match m:
     case a if False:
         reveal_type(a)  # E: Statement is unreachable \
-                        # N: Revealed type is "builtins.str"
+                        # N: Revealed type is "Never"
 
 [case testMatchRedefiningPatternGuard]
 m: str