Expand heuristic for inferring Parameters from a Callable

Author: A5rocks

mypy/checkexpr.py

@@ -2278,6 +2278,22 @@ def get_arg_infer_passes(
                     #   run(test, 1, 2)
                     # we will use `test` for inference, since it will allow to infer also
                     # argument *names* for P <: [x: int, y: int].
+                    if isinstance(p_actual, UnionType):
+                        new_items = []
+                        for item in p_actual.items:
+                            # narrow the union based on some approximations
+                            p_item = get_proper_type(item)
+                            if isinstance(p_item, CallableType) or (
+                                isinstance(p_item, Instance)
+                                and find_member("__call__", p_item, p_item, is_operator=True)
+                                is not None
+                            ):
+                                new_items.append(p_item)
+                                if len(new_items) == 2:
+                                    break
+
+                        if len(new_items) == 1:
+                            p_actual = new_items[0]
                     if isinstance(p_actual, Instance):
                         call_method = find_member("__call__", p_actual, p_actual, is_operator=True)
                         if call_method is not None:

test-data/unit/check-parameter-specification.test

@@ -2560,3 +2560,21 @@ def fn(f: MiddlewareFactory[P]) -> Capture[P]: ...
 
 reveal_type(fn(ServerErrorMiddleware))  # N: Revealed type is "__main__.Capture[[handler: Union[builtins.str, None] =, debug: builtins.bool =]]"
 [builtins fixtures/paramspec.pyi]
+
+[case testParamSpecInferenceWithAny]
+from typing_extensions import ParamSpec
+from typing import Any, Callable, Union
+
+P = ParamSpec("P")
+
+def into(f: Callable[P, None], *args: P.args, **kwargs: P.kwargs) -> None:
+    return None
+
+class C:
+    def f(self, y: bool = False, *, x: int = 42) -> None:
+        return None
+
+ex: Union[C, Any] = C()
+
+into(ex.f, x=-1)
+[builtins fixtures/paramspec.pyi]