Fix crash with tuple unpack inside TypeVar default

Author: cdce8p

mypy/typeops.py

@@ -113,6 +113,8 @@ def tuple_fallback(typ: TupleType) -> Instance:
                 and unpacked_type.type.fullname == "builtins.tuple"
             ):
                 items.append(unpacked_type.args[0])
+            elif isinstance(unpacked_type, TupleType):
+                items.extend(unpacked_type.items)
             else:
                 raise NotImplementedError
         else:

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

@@ -42,6 +42,7 @@ T4 = TypeVar("T4", int, str, default=int)
 T5 = TypeVar("T5", default=S0)
 T6 = TypeVar("T6", bound=float, default=S1)
 T7 = TypeVar("T7", bound=List[Any], default=List[S0])
+T8 = TypeVar("T8", default=Tuple[int, Unpack[Tuple[int, str]]])
 
 P1 = ParamSpec("P1", default=[])
 P2 = ParamSpec("P2", default=...)