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[libc++] Support more GCC 15 type_traits builtins #137871

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[libc++] Support more GCC 15 type_traits builtins #137871

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[libc++] Support more GCC 15 type_traits builtins
frobtech Apr 29, 2025
36df21c
Merge branch 'main' into p/libcxx-gcc15-type-traits
frobtech May 2, 2025
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10 changes: 10 additions & 0 deletions libcxx/include/__type_traits/add_lvalue_reference.h
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Original file line number Diff line number Diff line change
Expand Up @@ -20,8 +20,18 @@ _LIBCPP_BEGIN_NAMESPACE_STD

#if __has_builtin(__add_lvalue_reference)

# if defined(_LIBCPP_COMPILER_GCC)
template <class _Tp>
struct __add_lvalue_reference_gcc {
using type = __add_lvalue_reference(_Tp);
};
Comment on lines +24 to +27
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We should just use add_lvalue_reference in the GCC branch in the end. I think it would be better to just disable this branch for the GCC upgrade and do a proper refactoring afterwards. You've probably just copy-pasted what I've done for remove_cvref, but that's a bit special, since we've got an internal type trait for versions before remove_cvref was added.

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GCC 15 has only just been released and GCC 14 is still in wide use and does not have the builtin.

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We only support the latest version of GCC.

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If there is no window where both 14 and 15 work, then you make it impossible for people to upgrade. I need to upgrade to a fixed libc++ today so that I can upgrade to GCC 15 tomorrow. I can't upgrade to GCC 15 today because I need first land an upgraded libc++.

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That's been the policy for years and nobody had a problem with it. Also, this is not the PR to discuss policy changes. If you think we should change our support policy feel free to write an RFC.

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I'm trying to understand your interpretation of the policy. In #81386 you made a change that made libc++ work with both GCC 14 (the latest release branch at the time) and with GCC 15, which was unreleased and not a stable branch at the time. So by the policy, that change should not have gone in because only 14 was supported then.

Now, on the day that GCC 15 is released, the policy becomes that only 15 is supported and 14 is no longer supported. But the reality on that day was that 15 did not work and 14 did. By a straightforward and consistent interpretation of the policy, #81386 should not have gone in. By the same policy, at the instant of the GCC 15 release, 15 was the sole supported version--but it was broken, making it impossible for anyone using libc++ to switch to 15 on that day.

AFAICT the policy as written is obviously unworkable because as of today, the time window of GCC support is 5 days. AFAICT the policy as written also applies to me but not to you.

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I'm not saying you should break GCC 14 with this patch. I'm saying you ahould just disable the problematic branches, and after we upgrade the CI to 15 we'll clean it up.


template <class _Tp>
using __add_lvalue_reference_t _LIBCPP_NODEBUG = typename __add_lvalue_reference_gcc<_Tp>::type;
# else
template <class _Tp>
using __add_lvalue_reference_t _LIBCPP_NODEBUG = __add_lvalue_reference(_Tp);
# endif // defined(_LIBCPP_COMPILER_GCC)

#else

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10 changes: 10 additions & 0 deletions libcxx/include/__type_traits/add_pointer.h
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Expand Up @@ -22,8 +22,18 @@ _LIBCPP_BEGIN_NAMESPACE_STD

#if !defined(_LIBCPP_WORKAROUND_OBJCXX_COMPILER_INTRINSICS) && __has_builtin(__add_pointer)

# if defined(_LIBCPP_COMPILER_GCC)
template <class _Tp>
struct __add_pointer_gcc {
using type = __add_pointer(_Tp);
};

template <class _Tp>
using __add_pointer_t _LIBCPP_NODEBUG = typename __add_pointer_gcc<_Tp>::type;
# else
template <class _Tp>
using __add_pointer_t _LIBCPP_NODEBUG = __add_pointer(_Tp);
# endif // defined(_LIBCPP_COMPILER_GCC)

#else
template <class _Tp, bool = __is_referenceable_v<_Tp> || is_void<_Tp>::value>
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10 changes: 10 additions & 0 deletions libcxx/include/__type_traits/add_rvalue_reference.h
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Original file line number Diff line number Diff line change
Expand Up @@ -20,8 +20,18 @@ _LIBCPP_BEGIN_NAMESPACE_STD

#if __has_builtin(__add_rvalue_reference)

# if defined(_LIBCPP_COMPILER_GCC)
template <class _Tp>
struct __add_rvalue_reference_gcc {
using type = __add_rvalue_reference(_Tp);
};

template <class _Tp>
using __add_rvalue_reference_t _LIBCPP_NODEBUG = typename __add_rvalue_reference_gcc<_Tp>::type;
# else
template <class _Tp>
using __add_rvalue_reference_t _LIBCPP_NODEBUG = __add_rvalue_reference(_Tp);
# endif // defined(_LIBCPP_COMPILER_GCC)

#else

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10 changes: 10 additions & 0 deletions libcxx/include/__type_traits/decay.h
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Expand Up @@ -26,8 +26,18 @@
_LIBCPP_BEGIN_NAMESPACE_STD

#if __has_builtin(__decay)
# if defined(_LIBCPP_COMPILER_GCC)
template <class _Tp>
struct __decay_gcc {
using type = __decay(_Tp);
};

template <class _Tp>
using __decay_t _LIBCPP_NODEBUG = typename __decay_gcc<_Tp>::type;
# else
template <class _Tp>
using __decay_t _LIBCPP_NODEBUG = __decay(_Tp);
# endif // defined(_LIBCPP_COMPILER_GCC)

template <class _Tp>
struct _LIBCPP_NO_SPECIALIZATIONS decay {
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6 changes: 6 additions & 0 deletions libcxx/include/__type_traits/remove_all_extents.h
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Expand Up @@ -24,8 +24,14 @@ struct _LIBCPP_NO_SPECIALIZATIONS remove_all_extents {
using type _LIBCPP_NODEBUG = __remove_all_extents(_Tp);
};

# if defined(_LIBCPP_COMPILER_GCC)
template <class _Tp>
using __remove_all_extents_t = typename remove_all_extents<_Tp>::type;
# else
template <class _Tp>
using __remove_all_extents_t _LIBCPP_NODEBUG = __remove_all_extents(_Tp);
# endif // defined(_LIBCPP_COMPILER_GCC)

#else
template <class _Tp>
struct remove_all_extents {
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6 changes: 6 additions & 0 deletions libcxx/include/__type_traits/remove_extent.h
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Expand Up @@ -24,8 +24,14 @@ struct _LIBCPP_NO_SPECIALIZATIONS remove_extent {
using type _LIBCPP_NODEBUG = __remove_extent(_Tp);
};

# if defined(_LIBCPP_COMPILER_GCC)
template <class _Tp>
using __remove_extent_t = typename remove_extent<_Tp>::type;
# else
template <class _Tp>
using __remove_extent_t _LIBCPP_NODEBUG = __remove_extent(_Tp);
# endif // defined(_LIBCPP_COMPILER_GCC)

#else
template <class _Tp>
struct remove_extent {
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