Fix crash for callable with *args and suffix against Any (#18781)

Fixes #18780

Fix is trivial: handle a missing case. Note I re-use
`flatten_nested_tuples()` out of laziness. In theory, there should be at
most one level of nesting at this point, after which we should put an
assert (and IIRC we do something like this in other places). But I think
it is not worth the effort here, as this is a quite niche edge case
anyway.

mypy/constraints.py

@@ -53,6 +53,7 @@
     UnionType,
     UnpackType,
     find_unpack_in_list,
+    flatten_nested_tuples,
     get_proper_type,
     has_recursive_types,
     has_type_vars,
@@ -1347,7 +1348,9 @@ def visit_type_alias_type(self, template: TypeAliasType) -> list[Constraint]:
 
     def infer_against_any(self, types: Iterable[Type], any_type: AnyType) -> list[Constraint]:
         res: list[Constraint] = []
-        for t in types:
+        # Some items may be things like `*Tuple[*Ts, T]` for example from callable types with
+        # suffix after *arg, so flatten them.
+        for t in flatten_nested_tuples(types):
             if isinstance(t, UnpackType):
                 if isinstance(t.type, TypeVarTupleType):
                     res.append(Constraint(t.type, self.direction, any_type))

mypy/types.py

@@ -3726,7 +3726,7 @@ def find_unpack_in_list(items: Sequence[Type]) -> int | None:
     return unpack_index
 
 
-def flatten_nested_tuples(types: Sequence[Type]) -> list[Type]:
+def flatten_nested_tuples(types: Iterable[Type]) -> list[Type]:
     """Recursively flatten TupleTypes nested with Unpack.
 
     For example this will transform

test-data/unit/check-typevar-tuple.test

@@ -2606,3 +2606,15 @@ def test(xs: tuple[Unpack[Ts]], xsi: tuple[int, Unpack[Ts]]) -> None:
     reveal_type(join(xsi, ai))  # N: Revealed type is "Tuple[builtins.int, Unpack[builtins.tuple[Any, ...]]]"
     reveal_type(join(ai, xsi))  # N: Revealed type is "Tuple[builtins.int, Unpack[builtins.tuple[Any, ...]]]"
 [builtins fixtures/tuple.pyi]
+
+[case testTypeVarTupleInferAgainstAnyCallableSuffix]
+from typing import Any, Callable, TypeVar, TypeVarTuple
+
+Ts = TypeVarTuple("Ts")
+R = TypeVar("R")
+def deco(func: Callable[[*Ts, int], R]) -> Callable[[*Ts], R]:
+    ...
+
+untyped: Any
+reveal_type(deco(untyped))  # N: Revealed type is "def (*Any) -> Any"
+[builtins fixtures/tuple.pyi]