Fix narrowing of bool literal match subjects

mypy/typeops.py

@@ -1153,6 +1153,12 @@ class Status(Enum):
     if isinstance(typ, Instance) and (
         target_fullname is None or typ.type.fullname == target_fullname
     ):
+        if typ.last_known_value is not None:
+            # The expression is already known to hold this specific value, so keep it: expanding
+            # to all members of the sum type would forget it and break narrowing, e.g. matching a
+            # ``False`` literal subject against ``case False`` would not be exhaustive.
+            return typ.last_known_value
+
         if typ.type.fullname == "builtins.bool":
             return UnionType([LiteralType(True, typ), LiteralType(False, typ)])
 

test-data/unit/check-python310.test

@@ -50,6 +50,34 @@ def foo(m: bool, x: int):
             reveal_type(m)  # N: Revealed type is "builtins.bool"
 [builtins fixtures/primitives.pyi]
 
+[case testMatchBoolLiteralSubjectIsExhaustive]
+# flags: --strict-equality --warn-unreachable
+
+def f() -> int:
+    match False:
+        case True:
+            return 1  # E: Statement is unreachable
+        case False:
+            return 0
+
+def g(b: bool) -> int:
+    match b:
+        case True:
+            return 1
+        case False:
+            return 0
+
+def h() -> int:
+    match False:
+        case True:
+            return 1  # E: Statement is unreachable
+        case False:
+            return 0
+        case other:
+            reveal_type(other)  # E: Statement is unreachable
+            return 2
+[builtins fixtures/primitives.pyi]
+
 [case testMatchLiteralPatternUnreachable]
 # flags: --strict-equality --warn-unreachable
 m: int