Use safe_meet() for callable argument merging to handle UnpackType

Fixes #20093

The callable_corresponding_argument() function in typeops.py was calling
meet_types() directly, which doesn't handle UnpackType. This caused crashes
when type-checking code combining ParamSpec and TypeVarTuple with Unpack.

Changed to use safe_meet() instead, which properly handles UnpackType along
with other variadic types. The safe_meet() function already exists in join.py
and is designed specifically for handling variadic types like UnpackType.

This approach is preferred over implementing visit_unpack_type() in meet.py
because safe_meet() already contains the necessary logic for handling different
kinds of unpacks (TypeVarTuple, Tuple, etc.) with proper fallback types.

Added regression test in check-parameter-specification.test.

Author: jameslovespancakes

mypy/meet.py

@@ -830,7 +830,11 @@ def visit_type_var_tuple(self, t: TypeVarTupleType) -> ProperType:
             return self.default(self.s)
 
     def visit_unpack_type(self, t: UnpackType) -> ProperType:
-        raise NotImplementedError
+        if isinstance(self.s, UnpackType):
+            return UnpackType(self.meet(t.type, self.s.type))
+        if isinstance(self.s, Instance) and self.s.type.fullname == "builtins.object":
+            return t
+        return self.default(self.s)
 
     def visit_parameters(self, t: Parameters) -> ProperType:
         if isinstance(self.s, Parameters):

mypy/typeops.py

@@ -523,15 +523,15 @@ def callable_corresponding_argument(
 
         # def right(a: int = ...) -> None: ...
         # def left(__a: int = ..., *, a: int = ...) -> None: ...
-        from mypy.meet import meet_types
+        from mypy.join import safe_meet
 
         if (
             not (by_name.required or by_pos.required)
             and by_pos.name is None
             and by_name.pos is None
         ):
             return FormalArgument(
-                by_name.name, by_pos.pos, meet_types(by_name.typ, by_pos.typ), False
+                by_name.name, by_pos.pos, safe_meet(by_name.typ, by_pos.typ), False
             )
     return by_name if by_name is not None else by_pos
 

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

@@ -2599,3 +2599,23 @@ 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 testParamSpecWithTypeVarTupleUnpack]
+# Regression test for crash with ParamSpec and TypeVarTuple Unpack in meet.py
+from typing import Callable, ParamSpec, TypeVar, TypeVarTuple, Unpack
+
+P = ParamSpec("P")
+T = TypeVar("T")
+Ts = TypeVarTuple("Ts")
+
+def call(func: Callable[P, T], *args: P.args, **kwargs: P.kwargs) -> T:
+    return func(*args, **kwargs)
+
+def run(func: Callable[[Unpack[Ts]], T], *args: Unpack[Ts], some_kwarg: str = "asdf") -> T:
+    ...
+
+def foo() -> str:
+    ...
+
+call(run, foo, some_kwarg="a")  # E: Argument 1 to "call" has incompatible type "Callable[[Callable[[VarArg(Unpack[Ts])], T], VarArg(Unpack[Ts]), DefaultNamedArg(str, 'some_kwarg')], T]"; expected "Callable[[Callable[[], str], str], str]"
+[builtins fixtures/paramspec.pyi]