Re: C++ PATCH to fix bogus error with constexpr and empty class (PR c++/81933)

[Jason Merrill <jason at redhat dot com>]

OK, thanks.

On Mon, Jan 22, 2018 at 10:44 AM, Marek Polacek <polacek@redhat.com> wrote:
> On Mon, Jan 22, 2018 at 09:40:50AM -0500, Jason Merrill wrote:
>> This seems like a workaround rather than a fix; we should fix
>> split_nonconstant_init to work properly rather than override it.
>> Specifically, it seems odd to return true if num_elts is 0; we
>> shouldn't override DECL_INITIAL if we didn't actually split anything
>> out.
>
> Ah, ok, here's a patch for that, then.  Thanks.
>
> Bootstrapped/regtested on x86_64-linux, ok for trunk?
>
> 2018-01-22  Marek Polacek  <polacek@redhat.com>
>
>         PR c++/81933
>         * typeck2.c (split_nonconstant_init_1): Return false if we didn't
>         split out anything.
>
>         * g++.dg/cpp1y/constexpr-empty4.C: New test.
>
> diff --git gcc/cp/typeck2.c gcc/cp/typeck2.c
> index 8d933257f5f..8cb0e8811d3 100644
> --- gcc/cp/typeck2.c
> +++ gcc/cp/typeck2.c
> @@ -725,6 +725,11 @@ split_nonconstant_init_1 (tree dest, tree init)
>
>    /* The rest of the initializer is now a constant. */
>    TREE_CONSTANT (init) = 1;
> +
> +  /* We didn't split out anything.  */
> +  if (num_split_elts == 0)
> +    return false;
> +
>    return complete_p && complete_ctor_at_level_p (TREE_TYPE (init),
>                                                  num_split_elts, inner_type);
>  }
> diff --git gcc/testsuite/g++.dg/cpp1y/constexpr-empty4.C gcc/testsuite/g++.dg/cpp1y/constexpr-empty4.C
> index e69de29bb2d..059220f8268 100644
> --- gcc/testsuite/g++.dg/cpp1y/constexpr-empty4.C
> +++ gcc/testsuite/g++.dg/cpp1y/constexpr-empty4.C
> @@ -0,0 +1,51 @@
> +// PR c++/81933
> +// { dg-do compile { target c++14 } }
> +
> +namespace std {
> +template <typename _Tp> struct __decay_and_strip { typedef _Tp __type; };
> +template <int> struct enable_if { typedef int type; };
> +template <typename _Head> struct _Head_base {
> +  constexpr _Head_base(_Head) {}
> +};
> +template <unsigned long, typename...> struct _Tuple_impl;
> +template <unsigned long _Idx, typename _Head, typename... _Tail>
> +struct _Tuple_impl<_Idx, _Head, _Tail...> : _Tuple_impl<1, _Tail...>, // { dg-warning "direct base" }
> +                                            _Head_base<_Head> {
> +  typedef _Tuple_impl<1, _Tail...> _Inherited;
> +  typedef _Head_base<_Head> _Base;
> +  constexpr _Tuple_impl(_Head __head, _Tail... __tail)
> +      : _Inherited(__tail...), _Base(__head) {}
> +  _Tuple_impl(const _Tuple_impl &) = default;
> +  _Tuple_impl(_Tuple_impl &&);
> +};
> +template <unsigned long _Idx, typename _Head>
> +struct _Tuple_impl<_Idx, _Head> : _Head_base<_Head> {
> +  typedef _Head_base<_Head> _Base;
> +  constexpr _Tuple_impl(_Head __head) : _Base(__head) {}
> +};
> +template <int> struct _TC {
> +  static constexpr bool _NotSameTuple() { return true; }
> +};
> +template <typename... _Elements> class tuple : _Tuple_impl<0, _Elements...> {
> +  typedef _Tuple_impl<0, _Elements...> _Inherited;
> +
> +public:
> +  template <typename... _UElements,
> +            enable_if<_TC<1>::_NotSameTuple()>::type = false>
> +  constexpr tuple(_UElements... __elements) : _Inherited(__elements...) {}
> +  tuple(const tuple &) = default;
> +};
> +template <typename... _Elements>
> +constexpr tuple<typename __decay_and_strip<_Elements>::__type...>
> +    make_tuple(_Elements... __args) {
> +  typedef tuple<typename __decay_and_strip<_Elements>::__type...> __result_type;
> +  return __result_type(__args...);
> +}
> +}
> +struct any_udt {};
> +template <typename... Tuples> constexpr auto flatten(Tuples... tuples) {
> +  auto all = std::make_tuple(tuples...);
> +  auto flat(all);
> +  return flat;
> +}
> +constexpr auto fail = flatten(any_udt{}, any_udt{});
>
>         Marek