Try another approach to TypeGuard

Author: ilevkivskyi

mypy/binder.py

@@ -20,7 +20,6 @@
     ProperType,
     TupleType,
     Type,
-    TypeGuardedType,
     TypeOfAny,
     TypeType,
     TypeVarType,
@@ -278,10 +277,7 @@ def update_from_options(self, frames: list[Frame]) -> bool:
                 possible_types = []
                 for t in resulting_values:
                     assert t is not None
-                    if isinstance(t.type, TypeGuardedType):
-                        possible_types.append(t.type.type_guard)
-                    else:
-                        possible_types.append(t.type)
+                    possible_types.append(t.type)
                 if len(possible_types) == 1:
                     # This is to avoid calling get_proper_type() unless needed, as this may
                     # interfere with our (hacky) TypeGuard support.

mypy/meet.py

@@ -117,7 +117,10 @@ def narrow_declared_type(declared: Type, narrowed: Type) -> Type:
     """Return the declared type narrowed down to another type."""
     # TODO: check infinite recursion for aliases here.
     if isinstance(narrowed, TypeGuardedType):
-        # A type guard forces the new type even if it doesn't overlap the old.
+        # A type guard forces the new type even if it doesn't overlap the old...
+        if is_proper_subtype(declared, narrowed.type_guard, ignore_promotions=True):
+            # ...unless it is a proper supertype of declared type.
+            return declared
         return narrowed.type_guard
 
     original_declared = declared