pub fn relate_args_with_variances<I: Interner, R: TypeRelation<I>>(
relation: &mut R,
ty_def_id: I::DefId,
variances: I::VariancesOf,
a_arg: I::GenericArgs,
b_arg: I::GenericArgs,
fetch_ty_for_diag: bool,
) -> RelateResult<I, I::GenericArgs>