is_named_instance should be a TypeGuard (#13543)

Author: sobolevn

mypy/checker.py

@@ -4246,7 +4246,7 @@ def get_types_from_except_handler(self, typ: Type, n: Expression) -> list[Type]:
                 for item in typ.relevant_items()
                 for union_typ in self.get_types_from_except_handler(item, n)
             ]
-        elif isinstance(typ, Instance) and is_named_instance(typ, "builtins.tuple"):
+        elif is_named_instance(typ, "builtins.tuple"):
             # variadic tuple
             return [typ.args[0]]
         else:

mypy/types.py

@@ -17,7 +17,7 @@
     Union,
     cast,
 )
-from typing_extensions import Final, TypeAlias as _TypeAlias, overload
+from typing_extensions import Final, TypeAlias as _TypeAlias, TypeGuard, overload
 
 import mypy.nodes
 from mypy.bogus_type import Bogus
@@ -3172,7 +3172,7 @@ def strip_type(typ: Type) -> Type:
         return orig_typ
 
 
-def is_named_instance(t: Type, fullnames: str | tuple[str, ...]) -> bool:
+def is_named_instance(t: Type, fullnames: str | tuple[str, ...]) -> TypeGuard[Instance]:
     if not isinstance(fullnames, tuple):
         fullnames = (fullnames,)