Handle TypeVarTupleType when checking overload constraints (#16428)

Fixes #16427

The test case added in the first commit crashes.

The second commit addresses the crash - I don't know whether this fix is
correct, it just happens to stop the crash but it leads to a code branch
which just `continue`s out of a for loop iteration, so it might be
bypassing something it shouldn't. I don't completely understand it.

---------

Co-authored-by: Ivan Levkivskyi <levkivskyi@gmail.com>

mypy/constraints.py

@@ -949,7 +949,7 @@ def visit_instance(self, template: Instance) -> list[Constraint]:
             for item in actual.items:
                 if isinstance(item, UnpackType):
                     unpacked = get_proper_type(item.type)
-                    if isinstance(unpacked, TypeVarType):
+                    if isinstance(unpacked, TypeVarTupleType):
                         # Cannot infer anything for T from [T, ...] <: *Ts
                         continue
                     assert (

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

@@ -1789,6 +1789,24 @@ def test(a: Container[Any], b: Container[int], c: Container[str]):
     reveal_type(build(b, c))  # N: Revealed type is "__main__.Array[builtins.int, builtins.str]"
 [builtins fixtures/tuple.pyi]
 
+[case testTypeVarTupleOverloadArbitraryLength]
+from typing import Any, Tuple, TypeVar, TypeVarTuple, Unpack, overload
+
+T = TypeVar("T")
+Ts = TypeVarTuple("Ts")
+@overload
+def add(self: Tuple[Unpack[Ts]], other: Tuple[T]) -> Tuple[Unpack[Ts], T]:
+    ...
+@overload
+def add(self: Tuple[T, ...], other: Tuple[T, ...]) -> Tuple[T, ...]:
+    ...
+def add(self: Any, other: Any) -> Any:
+    ...
+def test(a: Tuple[int, str], b: Tuple[bool], c: Tuple[bool, ...]):
+    reveal_type(add(a, b))  # N: Revealed type is "Tuple[builtins.int, builtins.str, builtins.bool]"
+    reveal_type(add(b, c))  # N: Revealed type is "builtins.tuple[builtins.bool, ...]"
+[builtins fixtures/tuple.pyi]
+
 [case testTypeVarTupleIndexOldStyleNonNormalizedAndNonLiteral]
 from typing import Any, Tuple
 from typing_extensions import Unpack