[clang] pack indexing fails when index not laundered through class template

[vasama]

See https://godbolt.org/z/19EKoc86z or pack_indexing.zip.

The code contains this definition of __m_at_c:

#ifdef LAUNDER
template <unsigned long _Idx, unsigned long _Cnt>
struct ___m_at {
	static_assert(_Idx < _Cnt);
	static constexpr unsigned long value = _Idx + _Cnt * 0;
};

template <unsigned long _Np, class... _Ts>
using __m_at_c = _Ts...[___m_at<_Np, sizeof...(_Ts)>::value];
#else
template <unsigned long _Np, class... _Ts>
using __m_at_c = _Ts...[_Np];
#endif

When LAUNDER is defined, the pack indexing subscript is laundered through the class template ___m_at and compilation succeeds. When it is not defined, the subscript is used directly, and Clang produces the following diagnostic:

pack_indexing3.cpp:12:25: error: invalid index 1 for pack '_Ts' of size 1
   12 | using __m_at_c = _Ts...[_Np];
      |                         ^

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Lauri Vasama (vasama)

See https://godbolt.org/z/19EKoc86z or [pack_indexing.zip](https://github.com/user-attachments/files/17739083/pack_indexing.zip).

The code contains this definition of __m_at_c:

#ifdef LAUNDER
template <unsigned long _Idx, unsigned long _Cnt>
struct ___m_at {
	static_assert(_Idx < _Cnt);
	static constexpr unsigned long value = _Idx + _Cnt * 0;
};

template <unsigned long _Np, class... _Ts>
using __m_at_c = _Ts...[___m_at<_Np, sizeof...(_Ts)>::value];
#else
template <unsigned long _Np, class... _Ts>
using __m_at_c = _Ts...[_Np];
#endif

When LAUNDER is defined, the pack indexing subscript is laundered through the class template ___m_at and compilation succeeds. When it is not defined, the subscript is used directly, and Clang produces the following diagnostic:

pack_indexing3.cpp:12:25: error: invalid index 1 for pack '_Ts' of size 1
   12 | using __m_at_c = _Ts...[_Np];
      |                         ^

[ericniebler]

reduced repro here:

https://godbolt.org/z/WheP4r7qc

#ifdef LAUNDER
template <unsigned long Idx, unsigned long Cnt>
struct _m_at {
  static_assert(Idx < Cnt);
  static constexpr unsigned long value = Idx + Cnt * 0;
};

template <unsigned long Np, class... Ts>
using m_at_c = Ts...[_m_at<Np, sizeof...(Ts)>::value];
#else
template <unsigned long Np, class... Ts>
using m_at_c = Ts...[Np];
#endif

template <class...>
struct types;

template <class, class Up>
using msecond = Up;

template <class, long... Is>
struct indices;

template <class>
struct repack;

template <long... Idx>
struct repack<indices<long, Idx...>> {
  template <class... Ts>
  using f = types<m_at_c<Idx, Ts...>...>;
};

template <class... Args>
struct mdispatch_ {
  using Idx = __make_integer_seq<indices, long, sizeof...(Args)>;

  template <class... Ts>
  auto fn() -> msecond<
    int[sizeof...(Ts)],
    typename repack<Idx>::template f<Args...>>;
};

mdispatch_<int, int> d;