Narrowing for final type objects (#20661)

Fixes #20590
Fixes #15553
Closes #20627
Closes #15646

Author: hauntsaninja

mypy/typeops.py

@@ -1004,6 +1004,10 @@ def is_singleton_identity_type(typ: ProperType) -> bool:
         )
     if isinstance(typ, LiteralType):
         return typ.is_enum_literal() or isinstance(typ.value, bool)
+    if isinstance(typ, TypeType) and isinstance(typ.item, Instance) and typ.item.type.is_final:
+        return True
+    if isinstance(typ, CallableType) and typ.is_type_obj() and typ.type_object().is_final:
+        return True
     return False
 
 

test-data/unit/check-python310.test

@@ -3311,3 +3311,44 @@ def type_object(field_type: type[object]) -> str:
             return "bar"
     return ret
 [builtins fixtures/dict.pyi]
+
+
+[case testMatchFinalClass]
+from typing import final, NoReturn
+
+def assert_never(x: NoReturn) -> NoReturn: ...
+
+class Types:
+
+    @final
+    class A:
+        pass
+
+    @final
+    class B:
+        pass
+
+def foo(cls: type[Types.A | Types.B]) -> None:
+    match cls:
+        case Types.A:
+            reveal_type(cls)  # N: Revealed type is "type[__main__.Types.A]"
+        case Types.B:
+            reveal_type(cls)  # N: Revealed type is "type[__main__.Types.B]"
+        case _:
+            assert_never(cls)
+
+
+class TypeTypes:
+    A: type[Types.A] = Types.A
+    B: type[Types.B] = Types.B
+
+
+def bar(cls: type[Types.A | Types.B]) -> None:
+    match cls:
+        case TypeTypes.A:
+            reveal_type(cls)  # N: Revealed type is "type[__main__.Types.A]"
+        case TypeTypes.B:
+            reveal_type(cls)  # N: Revealed type is "type[__main__.Types.B]"
+        case _:
+            assert_never(cls)
+[builtins fixtures/tuple.pyi]