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[libc++] Enable the optimized _IsSame on GCC as well as Clang. 

However, there's a problem on both GCC and Clang: they can't mangle
`__is_same(T,U)` if it appears anywhere that affects mangling. That's
a hard error. And it turns out that GCC puts dependent return types
into the mangling more aggressively than Clang, so for GCC's benefit
we need to avoid using raw `_IsSame` in the return type of
`swap(tuple&, tuple&)`. Therefore, make `__all` into a named type
instead of an alias.

If we ever need to support a compiler without the __is_same builtin,
we can make this an alias template for `is_same<T,U>::type`.

Differential Revision: https://reviews.llvm.org/D115100
This commit is contained in:
Arthur O'Dwyer 2021-12-03 14:55:57 -05:00
parent 50187d2dd5
commit 5c0ea7488b
2 changed files with 9 additions and 29 deletions
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2
libcxx/include/__tuple
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@ -387,7 +387,7 @@ template <bool ..._Preds>
struct __all_dummy;
template <bool ..._Pred>
using __all = _IsSame<__all_dummy<_Pred...>, __all_dummy<((void)_Pred, true)...>>;
struct __all : _IsSame<__all_dummy<_Pred...>, __all_dummy<((void)_Pred, true)...>> {};
struct __tuple_sfinae_base {
template <template <class, class...> class _Trait,

36
libcxx/include/type_traits
View File

@ -550,8 +550,6 @@ template <bool _Bp, class _If, class _Then> using conditional_t = typename condi
// is_same
#if __has_keyword(__is_same)
template <class _Tp, class _Up>
struct _LIBCPP_TEMPLATE_VIS is_same : _BoolConstant<__is_same(_Tp, _Up)> { };
@ -560,36 +558,18 @@ template <class _Tp, class _Up>
inline constexpr bool is_same_v = __is_same(_Tp, _Up);
#endif
#else
template <class _Tp, class _Up> struct _LIBCPP_TEMPLATE_VIS is_same : public false_type {};
template <class _Tp> struct _LIBCPP_TEMPLATE_VIS is_same<_Tp, _Tp> : public true_type {};
#if _LIBCPP_STD_VER > 14
template <class _Tp, class _Up>
inline constexpr bool is_same_v = is_same<_Tp, _Up>::value;
#endif
#endif // __is_same
// _IsSame<T,U> has the same effect as is_same<T,U> but instantiates fewer types:
// is_same<A,B> and is_same<C,D> are guaranteed to be different types, but
// _IsSame<A,B> and _IsSame<C,D> are the same type (namely, false_type).
// Neither GCC nor Clang can mangle the __is_same builtin, so _IsSame
// mustn't be directly used anywhere that contributes to name-mangling
// (such as in a dependent return type).
template <class _Tp, class _Up>
using _IsSame = _BoolConstant<
#ifdef __clang__
__is_same(_Tp, _Up)
#else
is_same<_Tp, _Up>::value
#endif
>;
using _IsSame = _BoolConstant<__is_same(_Tp, _Up)>;
template <class _Tp, class _Up>
using _IsNotSame = _BoolConstant<
#ifdef __clang__
!__is_same(_Tp, _Up)
#else
!is_same<_Tp, _Up>::value
#endif
>;
using _IsNotSame = _BoolConstant<!__is_same(_Tp, _Up)>;
template <class _Tp>
using __test_for_primary_template = __enable_if_t<