22.10.8 Comparisons [comparisons] C++N4910:2022 (618) p745.cpp

# はじめに(Introduction)

N4910  Working Draft, Standard for Programming Language C++

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4910.pdf

n4910は、ISO/IEC JTC1 SC22 WG21の作業原案(Working Draft)です。
公式のISO/IEC 14882原本ではありません。
ISO/IEC JTC1 SC22 WG21では、可能な限り作業文書を公開し、幅広い意見を求めています。
一連の記事はコード断片をコンパイルできる形にする方法を検討してコンパイル、リンク、実行して、規格案の原文と処理系(g++, Clang++)との違いを確認し、技術内容を検討し、ISO/IEC JTC1 SC22 WG21にフィードバックするために用います。
また、CERT C++, MISRA C++等のコーディング標準のコード断片をコンパイルする際の参考にさせていただこうと考えています。CERT C++, MISRA C++が標準化の動きとの時間的なずれがあれば確認できれば幸いです。また、boostライブラリとの関連、Linux OS, TOPPERSカーネル、g++(GCC), clang++(LLVM）との関係も調査中です。
何か、抜け漏れ、耳より情報がありましたらおしらせくださると幸いです。

 <この項は書きかけです。順次追記します。>

## 背景(back ground)
C/C++でコンパイルエラーが出ると、途方にくれることがしばしばあります。
何回かに１回は、該当するエラーが検索できます。
ただ、条件が違っていて、そこでの修正方法では目的を達成しないこともしばしばです。いろいろな条件のコンパイルエラーとその対応方法について、広く記録することによって、いつか同じエラーに遭遇した時にやくに立つことを目指しています。

この半年の間で、三度、自分のネットでの記録に助けられたことがあります。
また過去に解決できなかった記録を１０種類以上、最近になって解決できたことがあります。それは、主に次の３つの情報に基づいています。

https://stackoverflow.com

http://ja.cppreference.com/

cpprefjp - C++日本語リファレンス

https://cpprefjp.github.io/

コンパイラの実装状況

https://cpprefjp.github.io/implementation-status.html

また
https://researchmap.jp/joub9b3my-1797580/#_1797580
に記載したサイトのお世話になっています。

## 作業方針(sequence)
Clang++では-std=c++03, C++2bの2種類
g++では-std=c++03, c++2bの２種類
でコンパイルし、

１）コンパイルエラーを収集する。
２）コンパイルエラーをなくす方法を検討する。
コンパイルエラーになる例を示すだけが目的のコードは、コンパイルエラーをなくすのではなく、コンパイルエラーの種類を収集するだけにする。
文法を示すのが目的のコード場合に、コンパイルエラーをなくすのに手間がかかる場合は、順次作業します。
３）リンクエラーをなくす方法を検討する。
文法を示すのが目的のコード場合に、リンクエラーをなくすのに手間がかかる場合は、順次作業します。
４）意味のある出力を作る。
コンパイル、リンクが通っても、意味のある出力を示そうとすると、コンパイル・リンクエラーが出て収拾できそうにない場合がある。順次作業します。

１）だけのものから４）まで進んだものと色々ある状態です。一歩でも前に進むご助言をお待ちしています。「検討事項」の欄に現状を記録するようにしています。

### C++N4910:2022 Standard Working Draft on ISO/IEC 14882(0) sample code compile list  

https://qiita.com/kaizen_nagoya/items/fc957ddddd402004bb91

### C++N4741, 2018 Standard Working Draft on ISO/IEC 14882 sample code compile list
https://qiita.com/kaizen_nagoya/items/3294c014044550896010

### C++N4606, 2016符号断片編纂一覧(example code compile list)
C++N4606, 2016 Working Draft 2016, ISO/IEC 14882, C++ standard(1) Example code compile list
https://qiita.com/kaizen_nagoya/items/df5d62c35bd6ed1c3d43/

### C++N3242, 2011 sample code compile list on clang++ and g++
https://qiita.com/kaizen_nagoya/items/685b5c1a2c17c1bf1318

## 編纂器(Compiler)
### clang++ --version
> Debian clang version 14.0.5-++20220610033153+c12386ae247c-1~exp1~20220610153237.151
Target: x86_64-pc-linux-gnu, Thread model: posix, InstalledDir: /usr/bin

### g++- --version
> g++ (GCC) 12.1.0  Copyright (C) 2022 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

# 22.10.8 Comparisons [comparisons] C++N4910:2022 (618) p745.cpp

## 算譜(source code)

```c++:p745.cpp
// C++N4910 Committee Draft, Standard for Programming Language C++
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/n4910.pdf
const char * n4910 = "22.10.8 Comparisons [comparisons] C++N4910:2022 (618) p745.cpp";
// Debian clang version 14.0.5-++20220610033153+c12386ae247c-
// g++ (GCC) 12.1.0 Copyright (C) 2022 Free Software Foundation, Inc.
// Edited by Dr. OGAWA Kiyoshi. Compile procedure and results record.
// C++N4910:2022 Standard Working Draft on ISO/IEC 14882(0) sample code compile list
// https://qiita.com/kaizen_nagoya/items/fc957ddddd402004bb91

#include "N4910.h"

using namespace std;

// 22.10.8.1 General [comparisons.general]
//  The library provides basic function object classes for all of the comparison operators in the language (7.6.9, 7.6.10).
//  For templates less, greater, less_equal, and greater_equal, the specializations for any pointer type yield a result consistent with the implementation-defined strict total order over pointers (3.27).
//  [Note 1: If a < b is well-defined for pointers a and b of type P, then (a < b) == less<P>()(a, b), (a > b) == greater<P>()(a, b), and so forth.
// For template specializations less<void>, greater<void>, less_equal<void>, and greater_equal<void>, if the call operator calls a built-in operator comparing pointers, the call operator yields a result consistent with the implementation-defined strict total order over pointers.2
template<> struct equal_to<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) == std::forward<U>(u));
    using is_transparent = unspecified;
};
template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
-> decltype(std::forward<T>(t) == std::forward<U>(u));
// 22.10.8.2 Class template equal_to [comparisons.equal.to]
template<class T = void> struct equal_to {
    constexpr bool operator()(const T& x, const T& y) const;
};
constexpr bool operator()(const T& x, const T& y) const;
// Returns: x == y.
// Returns: std::forward<T>(t) == std::forward<U>(u).
// 22.10.8.3 Class template not_equal_to [comparisons.not.equal.to]
template<class T = void> struct not_equal_to {
    constexpr bool operator()(const T& x, const T& y) const;
};
constexpr bool operator()(const T& x, const T& y) const;
// Returns: x != y.
template<> struct not_equal_to<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) != std::forward<U>(u));
    using is_transparent = unspecified;
};
template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
-> decltype(std::forward<T>(t) != std::forward<U>(u));
// Returns: std::forward<T>(t) != std::forward<U>(u).
// 22.10.8.4 Class template greater [comparisons.greater]
template<class T = void> struct greater {
    constexpr bool operator()(const T& x, const T& y) const;
};
constexpr bool operator()(const T& x, const T& y) const;
// Returns: x > y.
template<> struct greater<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) > std::forward<U>(u));
    using is_transparent = unspecified;
};
template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
-> decltype(std::forward<T>(t) > std::forward<U>(u));
// Returns: std::forward<T>(t) > std::forward<U>(u).
// 22.10.8.5 Class template less [comparisons.less]
template<class T = void> struct less {
    constexpr bool operator()(const T& x, const T& y) const;
    constexpr bool operator()(const T& x, const T& y) const;
};
// Returns: x < y.
template<> struct less<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) < std::forward<U>(u));
    using is_transparent = unspecified;
};
template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
-> decltype(std::forward<T>(t) < std::forward<U>(u));
// Returns: std::forward<T>(t) < std::forward<U>(u).
// 22.10.8.6 Class template greater_equal [comparisons.greater.equal]
template<class T = void> struct greater_equal {
    constexpr bool operator()(const T& x, const T& y) const;
};
constexpr bool operator()(const T& x, const T& y) const;
// Returns: x >= y.
template<> struct greater_equal<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) >= std::forward<U>(u));
    using is_transparent = unspecified;
};
template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
-> decltype(std::forward<T>(t) >= std::forward<U>(u));
// Returns: std::forward<T>(t) >= std::forward<U>(u).
// 22.10.8.7 Class template less_equal [comparisons.less.equal]
template<class T = void> struct less_equal {
    constexpr bool operator()(const T& x, const T& y) const;
};
constexpr bool operator()(const T& x, const T& y) const;
// Returns: x <= y.
template<> struct less_equal<void> {
    template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) <= std::forward<U>(u));
    using is_transparent = unspecified;
};
template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
-> decltype(std::forward<T>(t) <= std::forward<U>(u));
// Returns: std::forward<T>(t) <= std::forward<U>(u).
// 22.10.8.8 Class compare_three_way [comparisons.three.way]
namespace std {
struct compare_three_way {
    template<class T, class U>
    constexpr auto operator()(T&& t, U&& u) const;
    using is_transparent = unspecified;
};
}
template<class T, class U>
constexpr auto operator()(T&& t, U&& u) const;
// Constraints: T and U satisfy three_way_comparable_with.
// Preconditions: If the expression std::forward<T>(t) <=> std::forward<U>(u) results in a call to a built-in operator <=> comparing pointers of type P, the conversion sequences from both T and U to P are equality-preserving (18.2); otherwise, T and U model three_way_comparable_with.
// Effects: — If the expression std::forward<T>(t) <=> std::forward<U>(u) results in a call to a built-in operator <=> comparing pointers of type P, returns strong_ordering::less if (the converted value of) t precedes u in the implementation-defined strict total order over pointers (3.27), strong_ordering::greater if u precedes t, and otherwise strong_ordering::equal. — Otherwise, equivalent to:
return std::forward<T>(t) <=> std::forward<U>(u);
int main() {
    cout  <<  n4910 << endl;
    return EXIT_SUCCESS;
}
```

## 編纂・実行結果(compile and go)

```ubuntu:bash
$ clang++ p745.cpp -std=03 -o p745l -I. -Wall
In file included from p745.cpp:10:
In file included from ./N4910.h:11:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/atomic:38:
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/c++0x_warning.h:32:2: error: This file requires compiler and library support for the ISO C++ 2011 standard. This support must be enabled with the -std=c++11 or -std=gnu++11 compiler options.
#error This file requires compiler and library support \
 ^
p745.cpp:20:30: error: unknown type name 'constexpr'
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
                             ^
p745.cpp:20:57: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
                                                        ^
p745.cpp:20:64: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
                                                               ^
p745.cpp:20:40: error: storage class specified for a member declaration
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
                                       ^
p745.cpp:20:75: error: expected ';' at end of declaration list
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
                                                                          ^
                                                                          ;
p745.cpp:22:24: warning: alias declarations are a C++11 extension [-Wc++11-extensions]
using is_transparent = unspecified; };
                       ^
p745.cpp:22:24: error: unknown type name 'unspecified'
p745.cpp:23:28: error: unknown type name 'constexpr'
template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
                           ^
p745.cpp:23:55: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
                                                      ^
p745.cpp:23:62: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
                                                             ^
p745.cpp:23:68: error: non-member function cannot have 'const' qualifier
template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
                                                                   ^~~~~
p745.cpp:23:38: error: illegal storage class on function
template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
                                     ^
p745.cpp:23:73: error: expected ';' at end of declaration
template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
                                                                        ^
                                                                        ;
p745.cpp:24:5: error: cannot use arrow operator on a type
    -> decltype(std::forward<T>(t) == std::forward<U>(u));
    ^
p745.cpp:27:3: error: unknown type name 'constexpr'
  constexpr bool operator()(const T& x, const T& y) const;
  ^
p745.cpp:29:1: error: unknown type name 'constexpr'
constexpr bool operator()(const T& x, const T& y) const;
^
p745.cpp:29:33: error: unknown type name 'T'
constexpr bool operator()(const T& x, const T& y) const;
                                ^
p745.cpp:29:45: error: unknown type name 'T'
constexpr bool operator()(const T& x, const T& y) const;
                                            ^
p745.cpp:29:51: error: non-member function cannot have 'const' qualifier
constexpr bool operator()(const T& x, const T& y) const;
                                                  ^~~~~
p745.cpp:34:3: error: unknown type name 'constexpr'
  constexpr bool operator()(const T& x, const T& y) const;
  ^
p745.cpp:36:1: error: unknown type name 'constexpr'
constexpr bool operator()(const T& x, const T& y) const;
^
p745.cpp:36:33: error: unknown type name 'T'
constexpr bool operator()(const T& x, const T& y) const;
                                ^
p745.cpp:36:45: error: unknown type name 'T'
constexpr bool operator()(const T& x, const T& y) const;
                                            ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
5 warnings and 20 errors generated.
$ clang++ p745.cpp -std=2b -o p745l -I. -Wall
p745.cpp:19:19: error: redefinition of 'equal_to<>'
template<> struct equal_to<void> {
                  ^~~~~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/stl_function.h:488:12: note: previous definition is here
    struct equal_to<void>
           ^
p745.cpp:23:68: error: non-member function cannot have 'const' qualifier
template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
                                                                   ^~~~~
p745.cpp:23:43: error: overloaded 'operator()' must be a non-static member function
template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
                                          ^
p745.cpp:29:33: error: unknown type name 'T'
constexpr bool operator()(const T& x, const T& y) const;
                                ^
p745.cpp:29:45: error: unknown type name 'T'
constexpr bool operator()(const T& x, const T& y) const;
                                            ^
p745.cpp:29:51: error: non-member function cannot have 'const' qualifier
constexpr bool operator()(const T& x, const T& y) const;
                                                  ^~~~~
p745.cpp:36:33: error: unknown type name 'T'
constexpr bool operator()(const T& x, const T& y) const;
                                ^
p745.cpp:36:45: error: unknown type name 'T'
constexpr bool operator()(const T& x, const T& y) const;
                                            ^
p745.cpp:36:51: error: non-member function cannot have 'const' qualifier
constexpr bool operator()(const T& x, const T& y) const;
                                                  ^~~~~
p745.cpp:38:19: error: reference to 'not_equal_to' is ambiguous
template<> struct not_equal_to<void> {
                  ^
p745.cpp:33:33: note: candidate found by name lookup is 'not_equal_to'
template<class T = void> struct not_equal_to {
                                ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/stl_function.h:361:12: note: candidate found by name lookup is 'std::not_equal_to'
    struct not_equal_to : public binary_function<_Tp, _Tp, bool>
           ^
p745.cpp:41:24: error: unknown type name 'unspecified'
using is_transparent = unspecified; };
                       ^
p745.cpp:42:68: error: non-member function cannot have 'const' qualifier
template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
                                                                   ^~~~~
p745.cpp:42:43: error: overloaded 'operator()' must be a non-static member function
template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
                                          ^
p745.cpp:49:33: error: unknown type name 'T'
constexpr bool operator()(const T& x, const T& y) const;
                                ^
p745.cpp:49:45: error: unknown type name 'T'
constexpr bool operator()(const T& x, const T& y) const;
                                            ^
p745.cpp:49:51: error: non-member function cannot have 'const' qualifier
constexpr bool operator()(const T& x, const T& y) const;
                                                  ^~~~~
p745.cpp:51:19: error: reference to 'greater' is ambiguous
template<> struct greater<void> {
                  ^
p745.cpp:46:33: note: candidate found by name lookup is 'greater'
template<class T = void> struct greater {
                                ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/stl_function.h:371:12: note: candidate found by name lookup is 'std::greater'
    struct greater : public binary_function<_Tp, _Tp, bool>
           ^
p745.cpp:54:24: error: unknown type name 'unspecified'
using is_transparent = unspecified; };
                       ^
p745.cpp:55:68: error: non-member function cannot have 'const' qualifier
template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
                                                                   ^~~~~
fatal error: too many errors emitted, stopping now [-ferror-limit=]
20 errors generated.

$ g++ p745.cpp -std=03 -o p745g -I. -Wall
In file included from /usr/local/include/c++/12.1.0/atomic:38,
                 from N4910.h:11,
                 from p745.cpp:10:
/usr/local/include/c++/12.1.0/bits/c++0x_warning.h:32:2: error: #error This file requires compiler and library support for the ISO C++ 2011 standard. This support must be enabled with the -std=c++11 or -std=gnu++11 compiler options.
   32 | #error This file requires compiler and library support \
      |  ^~~~~
p745.cpp:20:30: warning: identifier 'constexpr' is a keyword in C++11 [-Wc++11-compat]
   20 |   template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      |                              ^~~~~~~~~
p745.cpp:21:8: warning: identifier 'decltype' is a keyword in C++11 [-Wc++11-compat]
   21 |     -> decltype(std::forward<T>(t) == std::forward<U>(u));
      |        ^~~~~~~~
p745.cpp:19:19: error: specialization of 'template<class _Tp> struct std::equal_to' in different namespace -fpermissive]
   19 | template<> struct equal_to<void> {
      |                   ^~~~~~~~~~~~~~
In file included from /usr/local/include/c++/12.1.0/string:48,
                 from /usr/local/include/c++/12.1.0/bits/locale_classes.h:40,
                 from /usr/local/include/c++/12.1.0/bits/ios_base.h:41,
                 from /usr/local/include/c++/12.1.0/ios:42,
                 from /usr/local/include/c++/12.1.0/ostream:38,
                 from /usr/local/include/c++/12.1.0/iostream:39,
                 from N4910.h:2:
/usr/local/include/c++/12.1.0/bits/stl_function.h:373:12: note:   from definition of 'template<class _Tp> struct std::equal_to'
  373 |     struct equal_to : public binary_function<_Tp, _Tp, bool>
      |            ^~~~~~~~
p745.cpp:19:19: error: explicit specialization of 'template<class _Tp> struct std::equal_to' outside its namespace must use a nested-name-specifier [-fpermissive]
   19 | template<> struct equal_to<void> {
      |                   ^~~~~~~~~~~~~~
p745.cpp:20:30: error: 'constexpr' does not name a type
   20 |   template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      |                              ^~~~~~~~~
p745.cpp:20:30: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p745.cpp:22:7: error: expected nested-name-specifier before 'is_transparent'
   22 | using is_transparent = unspecified; };
      |       ^~~~~~~~~~~~~~
p745.cpp:23:28: error: 'constexpr' does not name a type
   23 | template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      |                            ^~~~~~~~~
p745.cpp:23:28: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p745.cpp:27:3: error: 'constexpr' does not name a type
   27 |   constexpr bool operator()(const T& x, const T& y) const;
      |   ^~~~~~~~~
p745.cpp:27:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p745.cpp:29:1: error: 'constexpr' does not name a type
   29 | constexpr bool operator()(const T& x, const T& y) const;
      | ^~~~~~~~~
p745.cpp:29:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p745.cpp:34:3: error: 'constexpr' does not name a type
   34 |   constexpr bool operator()(const T& x, const T& y) const;
      |   ^~~~~~~~~
p745.cpp:34:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p745.cpp:36:1: error: 'constexpr' does not name a type
   36 | constexpr bool operator()(const T& x, const T& y) const;
      | ^~~~~~~~~
p745.cpp:36:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p745.cpp:38:19: error: 'not_equal_to' is not a class template
   38 | template<> struct not_equal_to<void> {
      |                   ^~~~~~~~~~~~
p745.cpp:38:36: error: template specifiers not specified in declaration of 'template<class T> struct not_equal_to'
   38 | template<> struct not_equal_to<void> {
      |                                    ^
p745.cpp:42:28: error: 'constexpr' does not name a type
   42 | template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      |                            ^~~~~~~~~
p745.cpp:42:28: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p745.cpp:47:3: error: 'constexpr' does not name a type
   47 |   constexpr bool operator()(const T& x, const T& y) const;
      |   ^~~~~~~~~
p745.cpp:47:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p745.cpp:49:1: error: 'constexpr' does not name a type
   49 | constexpr bool operator()(const T& x, const T& y) const;
      | ^~~~~~~~~
p745.cpp:49:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p745.cpp:51:19: error: 'greater' is not a class template
   51 | template<> struct greater<void> {
      |                   ^~~~~~~
p745.cpp:51:31: error: template specifiers not specified in declaration of 'template<class T> struct greate'
   51 | template<> struct greater<void> {
      |                               ^
p745.cpp:55:28: error: 'constexpr' does not name a type
   55 | template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      |                            ^~~~~~~~~
p745.cpp:55:28: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p745.cpp:60:3: error: 'constexpr' does not name a type
   60 |   constexpr bool operator()(const T& x, const T& y) const;
      |   ^~~~~~~~~
p745.cpp:60:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p745.cpp:61:1: error: 'constexpr' does not name a type
   61 | constexpr bool operator()(const T& x, const T& y) const;
      | ^~~~~~~~~
p745.cpp:61:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p745.cpp:64:19: error: 'less' is not a class template
   64 | template<> struct less<void> {
      |                   ^~~~
p745.cpp:64:28: error: template specifiers not specified in declaration of 'template<class T> struct less'
   64 | template<> struct less<void> {
      |                            ^
p745.cpp:68:28: error: 'constexpr' does not name a type
   68 | template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      |                            ^~~~~~~~~
p745.cpp:68:28: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p745.cpp:73:3: error: 'constexpr' does not name a type
   73 |   constexpr bool operator()(const T& x, const T& y) const;
      |   ^~~~~~~~~
p745.cpp:73:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p745.cpp:75:1: error: 'constexpr' does not name a type
   75 | constexpr bool operator()(const T& x, const T& y) const;
      | ^~~~~~~~~
p745.cpp:75:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p745.cpp:77:19: error: 'greater_equal' is not a class template
   77 | template<> struct greater_equal<void> {
      |                   ^~~~~~~~~~~~~
p745.cpp:77:37: error: template specifiers not specified in declaration of 'template<class T> struct greater_equal'
   77 | template<> struct greater_equal<void> {
      |                                     ^
p745.cpp:81:28: error: 'constexpr' does not name a type
   81 | template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      |                            ^~~~~~~~~
p745.cpp:81:28: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p745.cpp:86:3: error: 'constexpr' does not name a type
   86 |   constexpr bool operator()(const T& x, const T& y) const;
      |   ^~~~~~~~~
p745.cpp:86:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p745.cpp:88:1: error: 'constexpr' does not name a type
   88 | constexpr bool operator()(const T& x, const T& y) const;
      | ^~~~~~~~~
p745.cpp:88:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p745.cpp:90:19: error: 'less_equal' is not a class template
   90 | template<> struct less_equal<void> {
      |                   ^~~~~~~~~~
p745.cpp:90:34: error: template specifiers not specified in declaration of 'template<class T> struct less_equal'
   90 | template<> struct less_equal<void> {
      |                                  ^
p745.cpp:94:28: error: 'constexpr' does not name a type
   94 | template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      |                            ^~~~~~~~~
p745.cpp:94:28: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p745.cpp:101:9: error: 'constexpr' does not name a type
  101 |         constexpr auto operator()(T&& t, U&& u) const;
      |         ^~~~~~~~~
p745.cpp:101:9: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p745.cpp:102:7: error: expected nested-name-specifier before 'is_transparent'
  102 | using is_transparent = unspecified; };
      |       ^~~~~~~~~~~~~~
p745.cpp:105:3: error: 'constexpr' does not name a type
  105 |   constexpr auto operator()(T&& t, U&& u) const;
      |   ^~~~~~~~~
p745.cpp:105:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p745.cpp:109:1: error: expected unqualified-id before 'return'
  109 | return std::forward<T>(t) <=> std::forward<U>(u);
      | ^~~~~~

$ g++ p745.cpp -std=2b -o p745g -I. -Wall
p745.cpp:19:19: error: explicit specialization of 'template<class _Tp> struct std::equal_to' outside its namespace must use a nested-name-specifier [-fpermissive]
   19 | template<> struct equal_to<void> {
      |                   ^~~~~~~~~~~~~~
p745.cpp:19:19: error: redefinition of 'struct std::equal_to<void>'
In file included from /usr/local/include/c++/12.1.0/string:48,
                 from /usr/local/include/c++/12.1.0/bits/locale_classes.h:40,
                 from /usr/local/include/c++/12.1.0/bits/ios_base.h:41,
                 from /usr/local/include/c++/12.1.0/ios:42,
                 from /usr/local/include/c++/12.1.0/ostream:38,
                 from /usr/local/include/c++/12.1.0/iostream:39,
                 from N4910.h:2,
                 from p745.cpp:10:
/usr/local/include/c++/12.1.0/bits/stl_function.h:495:12: note: previous definition of 'struct std::equal_to<void>'
  495 |     struct equal_to<void>
      |            ^~~~~~~~~~~~~~
p745.cpp:24:8: error: non-member function 'constexpr decltype ((forward<T>(t) == forward<U>(u))) operator()(T&&, U&&)' cannot have cv-qualifier
   24 |     -> decltype(std::forward<T>(t) == std::forward<U>(u));
      |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
p745.cpp:23:43: error: 'constexpr decltype ((forward<T>(t) == forward<U>(u))) operator()(T&&, U&&)' must be a non-static member function
   23 | template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      |                                           ^~~~~~~~
p745.cpp:29:33: error: 'T' does not name a type
   29 | constexpr bool operator()(const T& x, const T& y) const;
      |                                 ^
p745.cpp:29:45: error: 'T' does not name a type
   29 | constexpr bool operator()(const T& x, const T& y) const;
      |                                             ^
p745.cpp:29:51: error: non-member function 'constexpr bool operator()(const int&, const int&)' cannot have cv-qualifier
   29 | constexpr bool operator()(const T& x, const T& y) const;
      |                                                   ^~~~~
p745.cpp:29:16: error: 'constexpr bool operator()(const int&, const int&)' must be a non-static member function
   29 | constexpr bool operator()(const T& x, const T& y) const;
      |                ^~~~~~~~
p745.cpp:36:33: error: 'T' does not name a type
   36 | constexpr bool operator()(const T& x, const T& y) const;
      |                                 ^
p745.cpp:36:45: error: 'T' does not name a type
   36 | constexpr bool operator()(const T& x, const T& y) const;
      |                                             ^
p745.cpp:36:51: error: non-member function 'constexpr bool operator()(const int&, const int&)' cannot have cv-qualifier
   36 | constexpr bool operator()(const T& x, const T& y) const;
      |                                                   ^~~~~
p745.cpp:36:16: error: 'constexpr bool operator()(const int&, const int&)' must be a non-static member function
   36 | constexpr bool operator()(const T& x, const T& y) const;
      |                ^~~~~~~~
p745.cpp:38:19: error: 'not_equal_to' is not a class template
   38 | template<> struct not_equal_to<void> {
      |                   ^~~~~~~~~~~~
p745.cpp:38:36: error: template specifiers not specified in declaration of 'template<class T> struct not_equal_to'
   38 | template<> struct not_equal_to<void> {
      |                                    ^
p745.cpp:43:8: error: non-member function 'constexpr decltype ((forward<T>(t) != forward<U>(u))) operator()(T&&, U&&)' cannot have cv-qualifier
   43 |     -> decltype(std::forward<T>(t) != std::forward<U>(u));
      |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
p745.cpp:42:43: error: 'constexpr decltype ((forward<T>(t) != forward<U>(u))) operator()(T&&, U&&)' must be a non-static member function
   42 | template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      |                                           ^~~~~~~~
p745.cpp:49:33: error: 'T' does not name a type
   49 | constexpr bool operator()(const T& x, const T& y) const;
      |                                 ^
p745.cpp:49:45: error: 'T' does not name a type
   49 | constexpr bool operator()(const T& x, const T& y) const;
      |                                             ^
p745.cpp:49:51: error: non-member function 'constexpr bool operator()(const int&, const int&)' cannot have cv-qualifier
   49 | constexpr bool operator()(const T& x, const T& y) const;
      |                                                   ^~~~~
p745.cpp:49:16: error: 'constexpr bool operator()(const int&, const int&)' must be a non-static member function
   49 | constexpr bool operator()(const T& x, const T& y) const;
      |                ^~~~~~~~
p745.cpp:51:19: error: 'greater' is not a class template
   51 | template<> struct greater<void> {
      |                   ^~~~~~~
p745.cpp:51:31: error: template specifiers not specified in declaration of 'template<class T> struct greate'
   51 | template<> struct greater<void> {
      |                               ^
p745.cpp:56:8: error: non-member function 'constexpr decltype ((forward<T>(t) > forward<U>(u))) operator()(T&&, U&&)' cannot have cv-qualifier
   56 |     -> decltype(std::forward<T>(t) > std::forward<U>(u));
      |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
p745.cpp:55:43: error: 'constexpr decltype ((forward<T>(t) > forward<U>(u))) operator()(T&&, U&&)' must be a non-static member function
   55 | template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      |                                           ^~~~~~~~
p745.cpp:61:16: error: 'constexpr bool less<T>::operator()(const T&, const T&) const' cannot be overloaded with 'constexpr bool less<T>::operator()(const T&, const T&) const'
   61 | constexpr bool operator()(const T& x, const T& y) const;
      |                ^~~~~~~~
p745.cpp:60:18: note: previous declaration 'constexpr bool less<T>::operator()(const T&, const T&) const'
   60 |   constexpr bool operator()(const T& x, const T& y) const;
      |                  ^~~~~~~~
p745.cpp:64:19: error: 'less' is not a class template
   64 | template<> struct less<void> {
      |                   ^~~~
p745.cpp:64:28: error: template specifiers not specified in declaration of 'template<class T> struct less'
   64 | template<> struct less<void> {
      |                            ^
p745.cpp:69:8: error: non-member function 'constexpr decltype ((forward<T>(t) < forward<U>(u))) operator()(T&&, U&&)' cannot have cv-qualifier
   69 |     -> decltype(std::forward<T>(t) < std::forward<U>(u));
      |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
p745.cpp:68:43: error: 'constexpr decltype ((forward<T>(t) < forward<U>(u))) operator()(T&&, U&&)' must be a non-static member function
   68 | template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      |                                           ^~~~~~~~
p745.cpp:75:33: error: 'T' does not name a type
   75 | constexpr bool operator()(const T& x, const T& y) const;
      |                                 ^
p745.cpp:75:45: error: 'T' does not name a type
   75 | constexpr bool operator()(const T& x, const T& y) const;
      |                                             ^
p745.cpp:75:51: error: non-member function 'constexpr bool operator()(const int&, const int&)' cannot have cv-qualifier
   75 | constexpr bool operator()(const T& x, const T& y) const;
      |                                                   ^~~~~
p745.cpp:75:16: error: 'constexpr bool operator()(const int&, const int&)' must be a non-static member function
   75 | constexpr bool operator()(const T& x, const T& y) const;
      |                ^~~~~~~~
p745.cpp:77:19: error: 'greater_equal' is not a class template
   77 | template<> struct greater_equal<void> {
      |                   ^~~~~~~~~~~~~
p745.cpp:77:37: error: template specifiers not specified in declaration of 'template<class T> struct greater_equal'
   77 | template<> struct greater_equal<void> {
      |                                     ^
p745.cpp:82:8: error: non-member function 'constexpr decltype ((forward<T>(t) >= forward<U>(u))) operator()(T&&, U&&)' cannot have cv-qualifier
   82 |     -> decltype(std::forward<T>(t) >= std::forward<U>(u));
      |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
p745.cpp:81:43: error: 'constexpr decltype ((forward<T>(t) >= forward<U>(u))) operator()(T&&, U&&)' must be a non-static member function
   81 | template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      |                                           ^~~~~~~~
p745.cpp:88:33: error: 'T' does not name a type
   88 | constexpr bool operator()(const T& x, const T& y) const;
      |                                 ^
p745.cpp:88:45: error: 'T' does not name a type
   88 | constexpr bool operator()(const T& x, const T& y) const;
      |                                             ^
p745.cpp:88:51: error: non-member function 'constexpr bool operator()(const int&, const int&)' cannot have cv-qualifier
   88 | constexpr bool operator()(const T& x, const T& y) const;
      |                                                   ^~~~~
p745.cpp:88:16: error: 'constexpr bool operator()(const int&, const int&)' must be a non-static member function
   88 | constexpr bool operator()(const T& x, const T& y) const;
      |                ^~~~~~~~
p745.cpp:90:19: error: 'less_equal' is not a class template
   90 | template<> struct less_equal<void> {
      |                   ^~~~~~~~~~
p745.cpp:90:34: error: template specifiers not specified in declaration of 'template<class T> struct less_equal'
   90 | template<> struct less_equal<void> {
      |                                  ^
p745.cpp:95:8: error: non-member function 'constexpr decltype ((forward<T>(t) <= forward<U>(u))) operator()(T&&, U&&)' cannot have cv-qualifier
   95 |     -> decltype(std::forward<T>(t) <= std::forward<U>(u));
      |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
p745.cpp:94:43: error: 'constexpr decltype ((forward<T>(t) <= forward<U>(u))) operator()(T&&, U&&)' must be a non-static member function
   94 | template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
      |                                           ^~~~~~~~
p745.cpp:99:12: error: redefinition of 'struct std::compare_three_way'
   99 |     struct compare_three_way {
      |            ^~~~~~~~~~~~~~~~~
In file included from /usr/local/include/c++/12.1.0/bits/char_traits.h:45,
                 from /usr/local/include/c++/12.1.0/ios:40:
/usr/local/include/c++/12.1.0/compare:546:10: note: previous definition of 'struct std::compare_three_way'
  546 |   struct compare_three_way
      |          ^~~~~~~~~~~~~~~~~
p745.cpp:105:43: error: non-member function 'constexpr auto operator()(T&&, U&&)' cannot have cv-qualifier
  105 |   constexpr auto operator()(T&& t, U&& u) const;
      |                                           ^~~~~
p745.cpp:105:18: error: 'constexpr auto operator()(T&&, U&&)' must be a non-static member function
  105 |   constexpr auto operator()(T&& t, U&& u) const;
      |                  ^~~~~~~~
p745.cpp:109:1: error: expected unqualified-id before 'return'
  109 | return std::forward<T>(t) <=> std::forward<U>(u);
      | ^~~~~~
```

## 検討事項(agenda)
コンパイルエラーを取るか、コンパイルエラーの理由を解説する。

<!-- リンクエラーの解説をする。clang++とg++の比較。意味のある出力。
コンパイルエラーを取るか、コンパイルエラーの理由を解説する。-->

# 参考資料(reference)


関連する自己参照以外は、こちらの先頭に移転。

C言語（C++)に対する誤解、曲解、無理解、爽快。

https://qiita.com/kaizen_nagoya/items/3f3992c9722c1cee2e3a

#include "N4910.h"

https://qiita.com/kaizen_nagoya/items/163a47cef7d6bb1c33a8

C++N4910資料の改善点

https://qiita.com/kaizen_nagoya/items/fb4ceb5d117a54641af3

dockerにclang

https://qiita.com/kaizen_nagoya/items/8829ffeee397eda50e80

docker gnu(gcc/g++) and llvm(clang/clang++)

https://qiita.com/drafts/059874ea39c4de64c0f7

コンパイル用shell script C版(clangとgcc)とC++版(clang++とg++)

https://qiita.com/kaizen_nagoya/items/74220c0577a512c2d7da


C++N4910:2022 tag follower 300人超えました。ありがとうございます。

https://qiita.com/kaizen_nagoya/items/a63fb96d351ac5c16ff7

astyle 使ってみた

https://qiita.com/kaizen_nagoya/items/3c9a87a781d53a1a23f3

DoCAP（ドゥーキャップ）って何ですか？

https://qiita.com/kaizen_nagoya/items/47e0e6509ab792c43327

小川メソッド　覚え（書きかけ）

https://qiita.com/kaizen_nagoya/items/3593d72eca551742df68

 ＜この記事は個人の過去の経験に基づく個人の感想です。現在所属する組織、業務とは関係がありません。＞

# 文書履歴(document history)
ver. 0.01 初稿 　20220731