Use a simpler heuristic: just check if callables are subtypes if we ignore argnames

Author: sterliakov

mypy/messages.py

@@ -2872,13 +2872,8 @@ def format_single(arg: Type) -> str:
         if (
             isinstance(left, CallableType)
             and isinstance(right, CallableType)
-            and is_subtype(left.ret_type, right.ret_type)
             and any(right.arg_names)
-            and len(right.arg_types) == len(left.arg_types)
-            and all(
-                format_single(aleft) == format_single(aright)
-                for aleft, aright in zip(left.arg_types, right.arg_types)
-            )
+            and is_subtype(left, right, ignore_pos_arg_names=True)
         ):
             min_verbosity = 1
 

test-data/unit/check-functions.test

@@ -3510,3 +3510,13 @@ class Child(Parent):
     method_without: Callable[["Child"], "Child"]
     method_with: Callable[["Child", str], "Child"]  # E: Incompatible types in assignment (expression has type "Callable[[str], Child]", base class "Parent" defined the type as "Callable[[Arg(str, 'param')], Parent]")
 [builtins fixtures/tuple.pyi]
+
+[case testDistinctFormattingUnion]
+from typing import Callable, Union
+from mypy_extensions import Arg
+
+def f(x: Callable[[Arg(int, 'x')], None]) -> None: pass
+
+y: Callable[[Union[int, str]], None]
+f(y)  # E: Argument 1 to "f" has incompatible type "Callable[[Union[int, str]], None]"; expected "Callable[[Arg(int, 'x')], None]"
+[builtins fixtures/tuple.pyi]