Fix: internal error caused by the generic type alias with an unpacked list

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-type-aliases.test

@@ -1237,6 +1237,20 @@ Ta2 = TypeAliasType("Ta2", None, type_params=(Unpack[Ts],))  # E: Free type vari
 
 [builtins fixtures/tuple.pyi]
 
+[case testGenericTypeAliasArgOfTypeAliasIsUnpackedTuple-xfail]
+from typing_extensions import TypeAlias
+from typing import TypeVarTuple, Unpack
+
+Ts = TypeVarTuple('Ts')
+
+TA: TypeAlias = tuple[Unpack[Ts]]
+
+# For Python 3.10, error: Invalid syntax
+# For other Python, error: "list[int]" cannot be unpacked (must be tuple or TypeVarTuple)
+v1: TA[*list[int]]
+v2: TA[Unpack[list[int]]]
+[builtins fixtures/tuple.pyi]
+
 [case testAliasInstanceNameClash]
 from lib import func
 class A: ...