Fix exhaustive match after unreachable cases

mypy/checker.py

@@ -5944,6 +5944,7 @@ def visit_match_stmt(self, s: MatchStmt) -> None:
             # The second pass narrows down the types and type checks bodies.
             unmatched_types: TypeMap = {s.subject: UninhabitedType()}
             for p, g, b in zip(s.patterns, s.guards, s.bodies):
+                case_is_unreachable = self.binder.is_unreachable()
                 current_subject_type = self.expr_checker.narrow_type_from_binder(
                     named_subject, subject_type
                 )
@@ -5996,8 +5997,11 @@ def visit_match_stmt(self, s: MatchStmt) -> None:
                             self.accept(b)
                     else:
                         self.accept(b)
-                self.push_type_map(else_map, from_assignment=False)
-                unmatched_types = else_map
+                if not case_is_unreachable:
+                    # Unreachable cases are still checked above, but they shouldn't revive
+                    # subject types that were already handled by earlier cases.
+                    self.push_type_map(else_map, from_assignment=False)
+                    unmatched_types = else_map
 
             if not is_unreachable_map(unmatched_types) and not self.current_node_deferred:
                 for typ in unmatched_types.values():

test-data/unit/check-python310.test

@@ -3625,6 +3625,21 @@ match b:
     case _:
         pass
 
+[case testExhaustiveMatchIgnoresUnreachableClassPattern]
+# flags: --enable-error-code exhaustive-match
+class A: pass
+class B: pass
+
+x: A | None
+
+match x:
+    case None:
+        pass
+    case A() as a:
+        pass
+    case B():
+        pass
+
 [case testNonExhaustiveMatchWithFlag]
 # flags: --enable-error-code exhaustive-match --strict-equality --warn-unreachable