[libcxx] Use __make_integer_seq builtin when available

[K-ballo]

Summary

Use the __make_integer_seq builtin, introduced by r252036, when available. This allows for an incredibly fast std::make_integer_sequence implementation.

[majnemer]

This LGTM but @mclow.lists or @EricWF should give the final say-so.

[EricWF]

@majnemer Will the compiler emit diagnostics comparable to the static asserts in the other implementation?

[majnemer]

@EricWF Yes, it will.

[EricWF]

The only issue I have with this is that we should still test both code patchs with clang so we can ensure the fallback implementation remains correct. I would change

#if __has_builtin(__make_integer_seq) && !defined(_LIBCPP_TESTING_FALLBACK_MAKE_INTEGER_SEQUENCE)

Then add a test called test/std/utilities/intseq/intseq.make/make_integer_seq_fallback.pass.cpp that contains:

#define _LIBCPP_TESTING_FALLBACK_MAKE_INTEGER_SEQUENCE
#include "make_integer_seq.pass.cpp"

After that these changes LGTM. @K-ballo I'm happy to make the changes as I commit this if your OK with that?

[K-ballo]

After that these changes LGTM. @K-ballo I'm happy to make the changes as I commit this if your OK with that?

@EricWF Go ahead, thank you!

[EricWF]

Committted in r255162. Thanks.

include/utility

struct _LIBCPP_TYPE_VIS_ONLY integer_sequence
     constexpr
     size_t
     size() noexcept { return sizeof...(_Ip); }
 };
 
 template<size_t... _Ip>
     using index_sequence = integer_sequence<size_t, _Ip...>;
 
+#if __has_builtin(__make_integer_seq)
+
+template <class _Tp, _Tp _Ep>
+struct __make_integer_sequence
+{
+    typedef __make_integer_seq<integer_sequence, _Tp, _Ep> type;
+};
+
+#else
+
 namespace __detail {
 
 template<typename _Tp, size_t ..._Extra> struct __repeat;
 template<typename _Tp, _Tp ..._Np, size_t ..._Extra> struct __repeat<integer_sequence<_Tp, _Np...>, _Extra...> {
   typedef integer_sequence<_Tp,
                            _Np...,
                            sizeof...(_Np) + _Np...,
                            2 * sizeof...(_Np) + _Np...,

 struct __make_integer_sequence
 {
     static_assert(is_integral<_Tp>::value,
                   "std::make_integer_sequence can only be instantiated with an integral type" );
     static_assert(0 <= _Ep, "std::make_integer_sequence input shall not be negative");
     typedef __make_integer_sequence_unchecked<_Tp, _Ep> type;
 };
 
+#endif
+
 template<class _Tp, _Tp _Np>
     using make_integer_sequence = typename __make_integer_sequence<_Tp, _Np>::type;
 
 template<size_t _Np>
     using make_index_sequence = make_integer_sequence<size_t, _Np>;
 
 template<class... _Tp>
     using index_sequence_for = make_index_sequence<sizeof...(_Tp)>;