When selecting an overload item for constraint template matching, treat any ParamSpec in the template as free

Author: sterliakov

mypy/constraints.py

@@ -1434,6 +1434,7 @@ def find_matching_overload_item(overloaded: Overloaded, template: CallableType)
             is_compat=mypy.subtypes.is_subtype,
             is_proper_subtype=False,
             ignore_return=True,
+            map_template_paramspec=True,
         ):
             return item
     # Fall back to the first item if we can't find a match. This is totally arbitrary --

mypy/subtypes.py

@@ -1576,6 +1576,7 @@ def is_callable_compatible(
     check_args_covariantly: bool = False,
     allow_partial_overlap: bool = False,
     strict_concatenate: bool = False,
+    map_template_paramspec: bool = False,
 ) -> bool:
     """Is the left compatible with the right, using the provided compatibility check?
 
@@ -1717,6 +1718,7 @@ def g(x: int) -> int: ...
         ignore_pos_arg_names=ignore_pos_arg_names,
         allow_partial_overlap=allow_partial_overlap,
         strict_concatenate_check=strict_concatenate_check,
+        map_template_paramspec=map_template_paramspec,
     )
 
 
@@ -1753,6 +1755,7 @@ def are_parameters_compatible(
     ignore_pos_arg_names: bool = False,
     allow_partial_overlap: bool = False,
     strict_concatenate_check: bool = False,
+    map_template_paramspec: bool = False,
 ) -> bool:
     """Helper function for is_callable_compatible, used for Parameter compatibility"""
     if right.is_ellipsis_args and not is_proper_subtype:
@@ -1781,6 +1784,8 @@ def are_parameters_compatible(
         # a subtype of erased template type.
         trivial_vararg_suffix = True
 
+    right_is_pspec = map_template_paramspec and right.param_spec() is not None
+
     # Match up corresponding arguments and check them for compatibility. In
     # every pair (argL, argR) of corresponding arguments from L and R, argL must
     # be "more general" than argR if L is to be a subtype of R.
@@ -1817,7 +1822,7 @@ def _incompatible(left_arg: FormalArgument | None, right_arg: FormalArgument | N
         _incompatible(left_star, right_star)
         and not trivial_vararg_suffix
         or _incompatible(left_star2, right_star2)
-    ):
+    ) and not right_is_pspec:
         return False
 
     # Phase 1b: Check non-star args: for every arg right can accept, left must
@@ -1848,7 +1853,7 @@ def _incompatible(left_arg: FormalArgument | None, right_arg: FormalArgument | N
     #           arguments. Get all further positional args of left, and make sure
     #           they're more general than the corresponding member in right.
     # TODO: handle suffix in UnpackType (i.e. *args: *Tuple[Ts, X, Y]).
-    if right_star is not None and not trivial_vararg_suffix:
+    if right_star is not None and not trivial_vararg_suffix and not right_is_pspec:
         # Synthesize an anonymous formal argument for the right
         right_by_position = right.try_synthesizing_arg_from_vararg(None)
         assert right_by_position is not None
@@ -1875,7 +1880,7 @@ def _incompatible(left_arg: FormalArgument | None, right_arg: FormalArgument | N
     # Phase 1d: Check kw args. Right has an infinite series of optional named
     #           arguments. Get all further named args of left, and make sure
     #           they're more general than the corresponding member in right.
-    if right_star2 is not None:
+    if right_star2 is not None and not right_is_pspec:
         right_names = {name for name in right.arg_names if name is not None}
         left_only_names = set()
         for name, kind in zip(left.arg_names, left.arg_kinds):

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

@@ -2603,3 +2603,26 @@ def run3(predicate: Callable[Concatenate[int, str, _P], None], *args: _P.args, *
                                            # E: Argument 1 has incompatible type "*tuple[Union[int, str], ...]"; expected "str" \
                                            # E: Argument 1 has incompatible type "*tuple[Union[int, str], ...]"; expected "_P.args"
 [builtins fixtures/paramspec.pyi]
+
+[case testParamSpecOverloadProtocol]
+from typing import ParamSpec, Protocol, TypeVar, overload
+
+_A_contra = TypeVar("_A_contra", bound=str, contravariant=True)
+_P = ParamSpec("_P")
+
+class Callback(Protocol[_A_contra, _P]):
+    def method(self, a: _A_contra, *args: _P.args, **kwargs: _P.kwargs) -> None: ...
+
+class Impl:
+    @overload
+    def method(self, a: int, b: str) -> None: ...
+    @overload
+    def method(self, a: str, b: int) -> None: ...
+    def method(self, a, b) -> None: ...
+
+def accepts_callback(cb: Callback[str, _P], *args: _P.args, **kwargs: _P.kwargs) -> int:
+    return 1
+
+a = accepts_callback(Impl(), 1)
+reveal_type(a)  # N: Revealed type is "builtins.int"
+[builtins fixtures/paramspec.pyi]