Fix: internal error caused by the generic type alias with an unpacked list (#20689)

Fix: #20549

---------

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

Author: hyperkai

mypy/expandtype.py

@@ -541,7 +541,9 @@ def visit_tuple_type(self, t: TupleType) -> Type:
             if isinstance(item, UnpackType):
                 unpacked = get_proper_type(item.type)
                 if isinstance(unpacked, Instance):
-                    assert unpacked.type.fullname == "builtins.tuple"
+                    # expand_type() may be called during semantic analysis, before invalid unpacks are fixed.
+                    if unpacked.type.fullname != "builtins.tuple":
+                        return t.partial_fallback.accept(self)
                     if t.partial_fallback.type.fullname != "builtins.tuple":
                         # If it is a subtype (like named tuple) we need to preserve it,
                         # this essentially mimics the logic in tuple_fallback().

test-data/unit/check-python311.test

@@ -321,3 +321,15 @@ def exc_name() -> None:
     except* RuntimeError as e:
         e = fn_exc(e)
 [builtins fixtures/exception.pyi]
+
+[case testGenericTypeAliasArgOfTypeAliasIsUnpackedTuple]
+from typing_extensions import TypeAlias
+from typing import TypeVarTuple, Unpack
+
+Ts = TypeVarTuple('Ts')
+TA: TypeAlias = tuple[Unpack[Ts]]
+
+v1: TA[*list[int]]  # E: "list[int]" cannot be unpacked (must be tuple or TypeVarTuple)
+
+v2: TA[Unpack[list[int]]]  # E: "list[int]" cannot be unpacked (must be tuple or TypeVarTuple)
+[builtins fixtures/tuple.pyi]