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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/110167] excessive compile time when optimizing
 std::to_array
Date: Thu, 08 Jun 2023 10:17:34 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110167

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The "obvious" alternative impl using a lambda expression is even slower:

template<unsigned long N>
struct array
{
  int data[N];
};

template<unsigned long...> struct seq { };

#ifndef USE_LAMBDA
template<unsigned long... Idx>
array<sizeof...(Idx)>
to_array_impl(int (&a)[sizeof...(Idx)], seq<Idx...>)
{
  return {{a[Idx]...}};
}
#endif

template<unsigned long N>
array<N>
to_array(int (&a)[N])
{
#ifndef USE_LAMBDA
  return to_array_impl(a, seq<__integer_pack(N)...>{});
#else
  return [&a]<unsigned long... I>(seq<I...>) {
    return array<N>{{ a[I]... }};
  }(seq<__integer_pack(N)...>{});
#endif
}

constexpr int S =3D 2000;
int f[S];

array<S> g(void) { return to_array(f); }=