Replace hard crash with typecheck error when subclass method has the …

…same name as type alias (#13015)

Work on #5425

Co-authored-by: Wesley Wright <wesleyw@dropbox.com>

mypy/checker.py

@@ -1600,7 +1600,8 @@ def check_method_override_for_base_with_name(
  else:
  original_type = NoneType()
  else:
- assert False, str(base_attr.node)
+ # Will always fail to typecheck below, since we know the node is a method
+ original_type = NoneType()
  if isinstance(original_node, (FuncDef, OverloadedFuncDef)):
  original_class_or_static = original_node.is_class or original_node.is_static
  elif isinstance(original_node, Decorator):

test-data/unit/check-classes.test

@@ -7300,3 +7300,31 @@ def identity_wrapper(func: FuncT) -> FuncT:
 def foo(self: Any) -> str:
  return ""
 
+[case testParentClassWithTypeAliasAndSubclassWithMethod]
+from typing import Any, Callable, TypeVar
+
+class Parent:
+ foo = Callable[..., int] 
+ class bar:
+ pass
+ import typing as baz
+ foobar = TypeVar("foobar")
+
+class Child(Parent):
+ def foo(self, val: int) -> int: # E: Signature of "foo" incompatible with supertype "Parent"
+ return val
+ def bar(self, val: str) -> str: # E: Signature of "bar" incompatible with supertype "Parent"
+ return val
+ def baz(self, val: float) -> float: # E: Signature of "baz" incompatible with supertype "Parent"
+ return val
+ def foobar(self) -> bool: # E: Signature of "foobar" incompatible with supertype "Parent"
+ return False
+
+x: Parent.foo = lambda: 5
+y: Parent.bar = Parent.bar()
+z: Parent.baz.Any = 1
+child = Child()
+a: int = child.foo(1)
+b: str = child.bar("abc")
+c: float = child.baz(3.4)
+d: bool = child.foobar()