[libc++] Fixes concepts overload resolution.

[Mordante]

Summary

D123182 fixes a bug in Clang's overload resolution. After it landed it
was discovered basic_format_arg's constructors contains this bug. This
fixes the bug in libc++, unblocking D123182.

The code has been tested in combination with D123182.

[royjacobson]

Thank you for the quick fix!

LGTM, small nit about the bool& change.

libcxx/include/__format/format_arg.h

163

I think here it's unnecessary because it's a specific type, but maybe you just preferred consistency?

[Mordante]

Thanks for the review!

libcxx/include/__format/format_arg.h

163

This matches the wording of http://eel.is/c++draft/format.arg closer. (Note I use a const reference instead of an rvalue reference. This was the wording of an earlier revision of the Standard, changing it to an rvalue reference is work in progress.)

[Mordante]

The fix is trivial so unblock the patch waiting for this.

libcxx/include/__format/format_arg.h

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
#endif
     long double __long_double;
     const char_type* __const_char_type_ptr;
     basic_string_view<char_type> __string_view;
     const void* __ptr;
     handle __handle;
   };
   __format::__arg_t __type_;
 
-  _LIBCPP_HIDE_FROM_ABI explicit basic_format_arg(bool __v) noexcept
+  _LIBCPP_HIDE_FROM_ABI explicit basic_format_arg(const bool& __v) noexcept
       : __boolean(__v), __type_(__format::__arg_t::__boolean) {}
 
   template <class _Tp>
-  _LIBCPP_HIDE_FROM_ABI explicit basic_format_arg(_Tp __v) noexcept
+  _LIBCPP_HIDE_FROM_ABI explicit basic_format_arg(const _Tp& __v) noexcept
       requires(same_as<_Tp, char_type> ||
                (same_as<_Tp, char> && same_as<char_type, wchar_t>))
       : __char_type(__v), __type_(__format::__arg_t::__char_type) {}
 
   template <__libcpp_signed_integer _Tp>
-  _LIBCPP_HIDE_FROM_ABI explicit basic_format_arg(_Tp __v) noexcept {
+  _LIBCPP_HIDE_FROM_ABI explicit basic_format_arg(const _Tp& __v) noexcept {
     if constexpr (sizeof(_Tp) <= sizeof(int)) {
       __int = static_cast<int>(__v);
       __type_ = __format::__arg_t::__int;
     } else if constexpr (sizeof(_Tp) <= sizeof(long long)) {
       __long_long = static_cast<long long>(__v);
       __type_ = __format::__arg_t::__long_long;
     }
 #ifndef _LIBCPP_HAS_NO_INT128
     else if constexpr (sizeof(_Tp) == sizeof(__int128_t)) {
       __i128 = __v;
       __type_ = __format::__arg_t::__i128;
     }
 #endif
     else
       static_assert(sizeof(_Tp) == 0, "An unsupported signed integer was used");
   }
 
   template <__libcpp_unsigned_integer _Tp>
-  _LIBCPP_HIDE_FROM_ABI explicit basic_format_arg(_Tp __v) noexcept {
+  _LIBCPP_HIDE_FROM_ABI explicit basic_format_arg(const _Tp& __v) noexcept {
     if constexpr (sizeof(_Tp) <= sizeof(unsigned)) {
       __unsigned = static_cast<unsigned>(__v);
       __type_ = __format::__arg_t::__unsigned;
     } else if constexpr (sizeof(_Tp) <= sizeof(unsigned long long)) {
       __unsigned_long_long = static_cast<unsigned long long>(__v);
       __type_ = __format::__arg_t::__unsigned_long_long;
     }
 #ifndef _LIBCPP_HAS_NO_INT128