Fix ParamSpec inference for callback protocols (#15986)

Fixes #15984

Fix is straightforward, `ParamSpec` inference special-casing should put
instances with `__call__` and callable types on same ground.

Author: ilevkivskyi

mypy/checkexpr.py

@@ -2185,6 +2185,10 @@ 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, Instance):
+                        call_method = find_member("__call__", p_actual, p_actual, is_operator=True)
+                        if call_method is not None:
+                            p_actual = get_proper_type(call_method)
                     if (
                         isinstance(p_actual, CallableType)
                         and not p_actual.variables

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

@@ -1824,3 +1824,18 @@ class C(Generic[P]): ...
 c: C[int, [int, str], str]  # E: Nested parameter specifications are not allowed
 reveal_type(c)  # N: Revealed type is "__main__.C[Any]"
 [builtins fixtures/paramspec.pyi]
+
+[case testParamSpecInferenceWithCallbackProtocol]
+from typing import Protocol, Callable, ParamSpec
+
+class CB(Protocol):
+    def __call__(self, x: str, y: int) -> None: ...
+
+P = ParamSpec('P')
+def g(fn: Callable[P, None], *args: P.args, **kwargs: P.kwargs) -> None: ...
+
+cb: CB
+g(cb, y=0, x='a')  # OK
+g(cb, y='a', x=0)  # E: Argument "y" to "g" has incompatible type "str"; expected "int" \
+                   # E: Argument "x" to "g" has incompatible type "int"; expected "str"
+[builtins fixtures/paramspec.pyi]