Fix crash with tuple unpack inside TypeVar default (#20456)

Fixes #20451

Author: cdce8p

mypy/mixedtraverser.py

@@ -58,6 +58,7 @@ def visit_type_alias_expr(self, o: TypeAliasExpr, /) -> None:
     def visit_type_var_expr(self, o: TypeVarExpr, /) -> None:
         super().visit_type_var_expr(o)
         o.upper_bound.accept(self)
+        o.default.accept(self)
         for value in o.values:
             value.accept(self)
 

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=...)