Fix crash on unpack call special-casing (#16381)

Fixes #16380

Fix is quite straightforward, what was an `assert` really needs to be an
`if`.

---------

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

mypy/checkexpr.py

@@ -2440,34 +2440,28 @@ def check_argument_types(
             # the suffices to the tuple, e.g. a single actual like
             # Tuple[Unpack[Ts], int]
             expanded_tuple = False
+            actual_kinds = [arg_kinds[a] for a in actuals]
             if len(actuals) > 1:
-                first_actual_arg_type = get_proper_type(arg_types[actuals[0]])
+                p_actual_type = get_proper_type(arg_types[actuals[0]])
                 if (
-                    isinstance(first_actual_arg_type, TupleType)
-                    and len(first_actual_arg_type.items) == 1
-                    and isinstance(first_actual_arg_type.items[0], UnpackType)
+                    isinstance(p_actual_type, TupleType)
+                    and len(p_actual_type.items) == 1
+                    and isinstance(p_actual_type.items[0], UnpackType)
+                    and actual_kinds == [nodes.ARG_STAR] + [nodes.ARG_POS] * (len(actuals) - 1)
                 ):
-                    # TODO: use walrus operator
-                    actual_types = [first_actual_arg_type.items[0]] + [
-                        arg_types[a] for a in actuals[1:]
-                    ]
-                    actual_kinds = [nodes.ARG_STAR] + [nodes.ARG_POS] * (len(actuals) - 1)
-
-                    # If we got here, the callee was previously inferred to have a suffix.
-                    assert isinstance(orig_callee_arg_type, UnpackType)
-                    assert isinstance(orig_callee_arg_type.type, ProperType) and isinstance(
-                        orig_callee_arg_type.type, TupleType
-                    )
-                    assert orig_callee_arg_type.type.items
-                    callee_arg_types = orig_callee_arg_type.type.items
-                    callee_arg_kinds = [nodes.ARG_STAR] + [nodes.ARG_POS] * (
-                        len(orig_callee_arg_type.type.items) - 1
-                    )
-                    expanded_tuple = True
+                    actual_types = [p_actual_type.items[0]] + [arg_types[a] for a in actuals[1:]]
+                    if isinstance(orig_callee_arg_type, UnpackType):
+                        p_callee_type = get_proper_type(orig_callee_arg_type.type)
+                        if isinstance(p_callee_type, TupleType):
+                            assert p_callee_type.items
+                            callee_arg_types = p_callee_type.items
+                            callee_arg_kinds = [nodes.ARG_STAR] + [nodes.ARG_POS] * (
+                                len(p_callee_type.items) - 1
+                            )
+                            expanded_tuple = True
 
             if not expanded_tuple:
                 actual_types = [arg_types[a] for a in actuals]
-                actual_kinds = [arg_kinds[a] for a in actuals]
                 if isinstance(orig_callee_arg_type, UnpackType):
                     unpacked_type = get_proper_type(orig_callee_arg_type.type)
                     if isinstance(unpacked_type, TupleType):

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

@@ -2185,3 +2185,25 @@ def test2(
                      # E: Missing named argument "b"
     return func(*args, **kwargs)
 [builtins fixtures/tuple.pyi]
+
+[case testUnpackTupleSpecialCaseNoCrash]
+from typing import Tuple, TypeVar
+from typing_extensions import Unpack
+
+T = TypeVar("T")
+
+def foo(*x: object) -> None: ...
+def bar(*x: int) -> None: ...
+def baz(*x: T) -> T: ...
+
+keys: Tuple[Unpack[Tuple[int, ...]]]
+
+foo(keys, 1)
+foo(*keys, 1)
+
+bar(keys, 1)  # E: Argument 1 to "bar" has incompatible type "Tuple[Unpack[Tuple[int, ...]]]"; expected "int"
+bar(*keys, 1)  # OK
+
+reveal_type(baz(keys, 1))  # N: Revealed type is "builtins.object"
+reveal_type(baz(*keys, 1))  # N: Revealed type is "builtins.int"
+[builtins fixtures/tuple.pyi]