https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81975
Bug ID: 81975
Summary: Unpacking two packs via alias erroneously complains
about mismatched argument packs
Product: gcc
Version: 7.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: barry.revzin at gmail dot com
Target Milestone: ---
Example slightly reduced from http://lists.isocpp.org/core/2017/08/2827.php:
template <class T, class U>
struct is_same;
struct make_list
{
template <typename... Ts>
using f = void;
};
template<template <typename, typename> class F, typename... Ts>
struct zip_with
{
#ifdef ALIAS
template <typename C, typename... Ss>
using continuation_t = typename C::template f<F<Ts, Ss>...>;
#else
template <typename C, typename... Ss>
struct continuation {
using type = typename C::template f<F<Ts, Ss>...>;
};
template <typename C, typename... Ss>
using continuation_t = typename continuation<C, Ss...>::type;
#endif
template<typename... Ss>
using f = continuation_t<make_list, Ss...>;
};
using T = zip_with<is_same>::f<>;
using U = zip_with<is_same, int>::f<int>;
using V = zip_with<is_same, int, int>::f<int, int>;
Compiling with -DALIAS works fine. Compiling without it leads to errors for T
and V (but not U!) about mismatched argument pack lengths while expanding
F<Ts,Ss>. The two formulations should be equivalent.
