Signal unreachability when narrowing with Type{Guard,Is}[Never]

Author: brianschubert

mypy/checker.py

@@ -5889,9 +5889,13 @@ def find_isinstance_check_helper(
                         # Also note that a care must be taken to unwrap this back at read places
                         # where we use this to narrow down declared type.
                         if node.callee.type_guard is not None:
+                            if isinstance(get_proper_type(node.callee.type_guard), UninhabitedType):
+                                return None, {}
                             return {expr: TypeGuardedType(node.callee.type_guard)}, {}
                         else:
                             assert node.callee.type_is is not None
+                            if isinstance(get_proper_type(node.callee.type_is), UninhabitedType):
+                                return None, {}
                             return conditional_types_to_typemaps(
                                 expr,
                                 *self.conditional_types_with_intersection(