22.6.2 Header <variant> synopsis [variant.syn] C++N4910:2022 (560) p701.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
https://cpprefjp.github.io
http://ja.cppreference.com/

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.

# 22.6.2 Header <variant> synopsis [variant.syn] C++N4910:2022 (560) p701.cpp

## 算譜(source code)

```c++:p701.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 = "22.6.2 Header <variant> synopsis [variant.syn] C++N4910:2022 (560) p701.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 <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cassert>
#include <coroutine>
#include <vector>
#include <complex>
#include <map>
#include <atomic>
#include <unordered_map>
#include <optional>
#include <memory>
#include <typeinfo>

using namespace std;

// Example 1
#include <compare>
namespace std {
// 22.6.3, class template variant
template<class... Types>
class variant;
// see 17.11.1
// 22.6.4, variant helper classes
template<class T> struct variant_size;
template<class T> struct variant_size<const T>;
template<class T> // not defined
inline constexpr size_t variant_size_v = variant_size<T>::value;
template<class... Types>
struct variant_size<variant<Types...>>;
template<size_t I, class T> struct variant_alternative; // not defined
template<size_t I, class T> struct variant_alternative<I, const T>;
template<size_t I, class T>
using variant_alternative_t = typename variant_alternative<I, T>::type;
template<size_t I, class... Types>
struct variant_alternative<I, variant<Types...>>;
inline constexpr size_t variant_npos = -1;
// 22.6.5, value access
template<class T, class... Types>
constexpr bool holds_alternative(const variant<Types...>&) noexcept;
template<size_t I, class... Types>
constexpr variant_alternative_t<I, variant<Types...>>& get(variant<Types...>&);
template<size_t I, class... Types>
constexpr variant_alternative_t<I, variant<Types...>>&& get(variant<Types...>&&);
template<size_t I, class... Types>
constexpr const variant_alternative_t<I, variant<Types...>>& get(const variant<Types...>&);
template<size_t I, class... Types>
constexpr const variant_alternative_t<I, variant<Types...>>&& get(const variant<Types...>&&);
template<class T, class... Types>
constexpr T& get(variant<Types...>&);
template<class T, class... Types>
constexpr T&& get(variant<Types...>&&);
template<class T, class... Types>
constexpr const T& get(const variant<Types...>&);
template<class T, class... Types>
constexpr const T&& get(const variant<Types...>&&);
template<size_t I, class... Types>
constexpr add_pointer_t<variant_alternative_t<I, variant<Types...>>>
get_if(variant<Types...>*) noexcept;
template<size_t I, class... Types>
constexpr add_pointer_t<const variant_alternative_t<I, variant<Types...>>>
get_if(const variant<Types...>*) noexcept;
template<class T, class... Types>
constexpr add_pointer_t<T>
get_if(variant<Types...>*) noexcept;
template<class T, class... Types>
constexpr add_pointer_t<const T>
get_if(const variant<Types...>*) noexcept;
// 22.6.6, relational operators
template<class... Types>
constexpr bool operator==(const variant<Types...>&, const variant<Types...>&);
template<class... Types>
constexpr bool operator!=(const variant<Types...>&, const variant<Types...>&);
template<class... Types>
constexpr bool operator<(const variant<Types...>&, const variant<Types...>&);
template<class... Types>
constexpr bool operator>(const variant<Types...>&, const variant<Types...>&);
template<class... Types>
constexpr bool operator<=(const variant<Types...>&, const variant<Types...>&);
template<class... Types>
constexpr bool operator>=(const variant<Types...>&, const variant<Types...>&);
template<class... Types> requires (three_way_comparable<Types> && ...)
constexpr common_comparison_category_t<compare_three_way_result_t<Types>...>
operator<=>(const variant<Types...>&, const variant<Types...>&);
// 22.6.7, visitation
template<class Visitor, class... Variants>
constexpr see_below visit(Visitor&&, Variants&&...);
template<class R, class Visitor, class... Variants>
constexpr R visit(Visitor&&, Variants&&...);
// 22.6.8, class monostate struct monostate;
// 22.6.9, monostate relational operators
constexpr bool operator==(monostate, monostate) noexcept;
constexpr strong_ordering operator<=>(monostate, monostate) noexcept;
// 22.6.10, specialized algorithms
template<class... Types>
constexpr void swap(variant<Types...>&, variant<Types...>&) noexcept(see_below);
// 22.6.11, class bad_variant_access
class bad_variant_access;
// 22.6.12, hash support
template<class T> struct hash;
template<class... Types> struct hash<variant<Types...>>;
template<> struct hash<monostate>;
}
int main() {
    cout  <<  n4910 << endl;
    return EXIT_SUCCESS;
}
```

## 編纂・実行結果(compile and go)

```ubuntu:bash
$ clang++ p701.cpp -std=03 -o p701l -I. -Wall
In file included from p701.cpp:19:
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 \
 ^
p701.cpp:31:15: warning: variadic templates are a C++11 extension [-Wc++11-extensions]
template<class... Types>
              ^
p701.cpp:38:10: error: unknown type name 'constexpr'
  inline constexpr size_t variant_size_v = variant_size<T>::value;
         ^
p701.cpp:38:20: warning: variable templates are a C++14 extension [-Wc++14-extensions]
  inline constexpr size_t variant_size_v = variant_size<T>::value;
                   ^
p701.cpp:38:3: warning: inline variables are a C++17 extension [-Wc++17-extensions]
  inline constexpr size_t variant_size_v = variant_size<T>::value;
  ^
p701.cpp:38:26: error: expected ';' at end of declaration
  inline constexpr size_t variant_size_v = variant_size<T>::value;
                         ^
                         ;
p701.cpp:38:27: error: C++ requires a type specifier for all declarations
  inline constexpr size_t variant_size_v = variant_size<T>::value;
                          ^
p701.cpp:38:57: error: use of undeclared identifier 'T'
  inline constexpr size_t variant_size_v = variant_size<T>::value;
                                                        ^
p701.cpp:39:15: warning: variadic templates are a C++11 extension [-Wc++11-extensions]
template<class... Types>
              ^
p701.cpp:40:39: error: a space is required between consecutive right angle brackets (use '> >')
  struct variant_size<variant<Types...>>;
                                      ^~
                                      > >
p701.cpp:44:33: warning: alias declarations are a C++11 extension [-Wc++11-extensions]
  using variant_alternative_t = typename variant_alternative<I, T>::type;
                                ^
p701.cpp:45:25: warning: variadic templates are a C++11 extension [-Wc++11-extensions]
template<size_t I, class... Types>
                        ^
p701.cpp:46:49: error: a space is required between consecutive right angle brackets (use '> >')
  struct variant_alternative<I, variant<Types...>>;
                                                ^~
                                                > >
p701.cpp:47:8: error: unknown type name 'constexpr'
inline constexpr size_t variant_npos = -1;
       ^
p701.cpp:47:1: warning: inline variables are a C++17 extension [-Wc++17-extensions]
inline constexpr size_t variant_npos = -1;
^
p701.cpp:47:24: error: expected ';' after top level declarator
inline constexpr size_t variant_npos = -1;
                       ^
                       ;
p701.cpp:49:24: warning: variadic templates are a C++11 extension [-Wc++11-extensions]
template<class T, class... Types>
                       ^
p701.cpp:50:3: error: unknown type name 'constexpr'
  constexpr bool holds_alternative(const variant<Types...>&) noexcept;
  ^
p701.cpp:50:61: error: expected ';' at end of declaration
  constexpr bool holds_alternative(const variant<Types...>&) noexcept;
                                                            ^
                                                            ;
p701.cpp:50:62: error: C++ requires a type specifier for all declarations
  constexpr bool holds_alternative(const variant<Types...>&) noexcept;
                                                             ^
p701.cpp:51:25: warning: variadic templates are a C++11 extension [-Wc++11-extensions]
template<size_t I, class... Types>
                        ^
p701.cpp:52:3: error: unknown type name 'constexpr'
  constexpr variant_alternative_t<I, variant<Types...>>& get(variant<Types...>&);
  ^
p701.cpp:52:54: error: a space is required between consecutive right angle brackets (use '> >')
  constexpr variant_alternative_t<I, variant<Types...>>& get(variant<Types...>&);
                                                     ^~
                                                     > >
p701.cpp:52:13: error: no variable template matches partial specialization
  constexpr variant_alternative_t<I, variant<Types...>>& get(variant<Types...>&);
            ^
p701.cpp:52:56: error: expected ';' at end of declaration
  constexpr variant_alternative_t<I, variant<Types...>>& get(variant<Types...>&);
                                                       ^
                                                       ;
p701.cpp:52:70: error: use of undeclared identifier 'Types'
  constexpr variant_alternative_t<I, variant<Types...>>& get(variant<Types...>&);
                                                                     ^
p701.cpp:52:58: error: C++ requires a type specifier for all declarations
  constexpr variant_alternative_t<I, variant<Types...>>& get(variant<Types...>&);
                                                         ^
p701.cpp:53:25: warning: variadic templates are a C++11 extension [-Wc++11-extensions]
template<size_t I, class... Types>
                        ^
p701.cpp:54:3: error: unknown type name 'constexpr'
  constexpr variant_alternative_t<I, variant<Types...>>&& get(variant<Types...>&&);
  ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
10 warnings and 20 errors generated.
$ clang++ p701.cpp -std=2b -o p701l -I. -Wall
p701.cpp:97:11: error: unknown type name 'see_below'
constexpr see_below visit(Visitor&&, Variants&&...); 
          ^
p701.cpp:102:27: error: unknown type name 'monostate'
constexpr bool operator==(monostate, monostate) noexcept;
                          ^
p701.cpp:102:38: error: unknown type name 'monostate'
constexpr bool operator==(monostate, monostate) noexcept;
                                     ^
p701.cpp:103:39: error: unknown type name 'monostate'
constexpr strong_ordering operator<=>(monostate, monostate) noexcept;
                                      ^
p701.cpp:103:50: error: unknown type name 'monostate'
constexpr strong_ordering operator<=>(monostate, monostate) noexcept;
                                                 ^
p701.cpp:106:70: error: use of undeclared identifier 'see_below'
constexpr void swap(variant<Types...>&, variant<Types...>&) noexcept(see_below); 
                                                                     ^
p701.cpp:112:24: error: use of undeclared identifier 'monostate'
template<> struct hash<monostate>;
                       ^
7 errors generated.

$ g++ p701.cpp -std=03 -o p701g -I. -Wall
In file included from /usr/local/include/c++/12.1.0/atomic:38,
                 from p701.cpp:19:
/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 \
      |  ^~~~~
p701.cpp:38:10: warning: identifier 'constexpr' is a keyword in C++11 [-Wc++11-compat]
   38 |   inline constexpr size_t variant_size_v = variant_size<T>::value;
      |          ^~~~~~~~~
p701.cpp:50:62: warning: identifier 'noexcept' is a keyword in C++11 [-Wc++11-compat]
   50 |   constexpr bool holds_alternative(const variant<Types...>&) noexcept;
      |                                                              ^~~~~~~~
p701.cpp:31:15: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   31 | template<class... Types>
      |               ^~~
p701.cpp:38:10: error: 'constexpr' does not name a type
   38 |   inline constexpr size_t variant_size_v = variant_size<T>::value;
      |          ^~~~~~~~~
p701.cpp:38:10: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:39:15: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   39 | template<class... Types>
      |               ^~~
p701.cpp:40:39: error: '>>' should be '> >' within a nested template argument list
   40 |   struct variant_size<variant<Types...>>;
      |                                       ^~
      |                                       > >
p701.cpp:44:3: error: expected unqualified-id before 'using'
   44 |   using variant_alternative_t = typename variant_alternative<I, T>::type;
      |   ^~~~~
p701.cpp:45:25: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   45 | template<size_t I, class... Types>
      |                         ^~~
p701.cpp:46:49: error: '>>' should be '> >' within a nested template argument list
   46 |   struct variant_alternative<I, variant<Types...>>;
      |                                                 ^~
      |                                                 > >
p701.cpp:47:8: error: 'constexpr' does not name a type
   47 | inline constexpr size_t variant_npos = -1;
      |        ^~~~~~~~~
p701.cpp:47:8: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:49:24: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   49 | template<class T, class... Types>
      |                        ^~~
p701.cpp:50:3: error: 'constexpr' does not name a type
   50 |   constexpr bool holds_alternative(const variant<Types...>&) noexcept;
      |   ^~~~~~~~~
p701.cpp:50:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:51:25: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   51 | template<size_t I, class... Types>
      |                         ^~~
p701.cpp:52:3: error: 'constexpr' does not name a type
   52 |   constexpr variant_alternative_t<I, variant<Types...>>& get(variant<Types...>&);
      |   ^~~~~~~~~
p701.cpp:52:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:53:25: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   53 | template<size_t I, class... Types>
      |                         ^~~
p701.cpp:54:3: error: 'constexpr' does not name a type
   54 |   constexpr variant_alternative_t<I, variant<Types...>>&& get(variant<Types...>&&);
      |   ^~~~~~~~~
p701.cpp:54:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:55:25: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   55 | template<size_t I, class... Types>
      |                         ^~~
p701.cpp:56:3: error: 'constexpr' does not name a type
   56 |   constexpr const variant_alternative_t<I, variant<Types...>>& get(const variant<Types...>&);
      |   ^~~~~~~~~
p701.cpp:56:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:57:25: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   57 | template<size_t I, class... Types>
      |                         ^~~
p701.cpp:58:3: error: 'constexpr' does not name a type
   58 |   constexpr const variant_alternative_t<I, variant<Types...>>&& get(const variant<Types...>&&);
      |   ^~~~~~~~~
p701.cpp:58:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:59:24: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   59 | template<class T, class... Types>
      |                        ^~~
p701.cpp:60:3: error: 'constexpr' does not name a type
   60 |   constexpr T& get(variant<Types...>&);
      |   ^~~~~~~~~
p701.cpp:60:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:61:24: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   61 | template<class T, class... Types>
      |                        ^~~
p701.cpp:62:3: error: 'constexpr' does not name a type
   62 |   constexpr T&& get(variant<Types...>&&);
      |   ^~~~~~~~~
p701.cpp:62:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:63:24: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   63 | template<class T, class... Types>
      |                        ^~~
p701.cpp:64:3: error: 'constexpr' does not name a type
   64 |   constexpr const T& get(const variant<Types...>&);
      |   ^~~~~~~~~
p701.cpp:64:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:65:24: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   65 | template<class T, class... Types>
      |                        ^~~
p701.cpp:66:3: error: 'constexpr' does not name a type
   66 |   constexpr const T&& get(const variant<Types...>&&);
      |   ^~~~~~~~~
p701.cpp:66:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:67:27: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   67 |   template<size_t I, class... Types>
      |                           ^~~
p701.cpp:68:5: error: 'constexpr' does not name a type
   68 |     constexpr add_pointer_t<variant_alternative_t<I, variant<Types...>>>
      |     ^~~~~~~~~
p701.cpp:68:5: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:70:27: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   70 |   template<size_t I, class... Types>
      |                           ^~~
p701.cpp:71:5: error: 'constexpr' does not name a type
   71 |     constexpr add_pointer_t<const variant_alternative_t<I, variant<Types...>>>
      |     ^~~~~~~~~
p701.cpp:71:5: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:73:26: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   73 |   template<class T, class... Types>
      |                          ^~~
p701.cpp:74:5: error: 'constexpr' does not name a type
   74 |     constexpr add_pointer_t<T>
      |     ^~~~~~~~~
p701.cpp:74:5: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:76:26: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   76 |   template<class T, class... Types>
      |                          ^~~
p701.cpp:77:5: error: 'constexpr' does not name a type
   77 |     constexpr add_pointer_t<const T>
      |     ^~~~~~~~~
p701.cpp:77:5: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:80:15: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   80 | template<class... Types>
      |               ^~~
p701.cpp:81:5: error: 'constexpr' does not name a type
   81 |     constexpr bool operator==(const variant<Types...>&, const variant<Types...>&);
      |     ^~~~~~~~~
p701.cpp:81:5: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:82:17: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   82 |   template<class... Types>
      |                 ^~~
p701.cpp:83:5: error: 'constexpr' does not name a type
   83 |     constexpr bool operator!=(const variant<Types...>&, const variant<Types...>&);
      |     ^~~~~~~~~
p701.cpp:83:5: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:84:17: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   84 |   template<class... Types>
      |                 ^~~
p701.cpp:85:5: error: 'constexpr' does not name a type
   85 |     constexpr bool operator<(const variant<Types...>&, const variant<Types...>&);
      |     ^~~~~~~~~
p701.cpp:85:5: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:86:17: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   86 |   template<class... Types>
      |                 ^~~
p701.cpp:87:5: error: 'constexpr' does not name a type
   87 |     constexpr bool operator>(const variant<Types...>&, const variant<Types...>&);
      |     ^~~~~~~~~
p701.cpp:87:5: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:88:17: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   88 |   template<class... Types>
      |                 ^~~
p701.cpp:89:5: error: 'constexpr' does not name a type
   89 |     constexpr bool operator<=(const variant<Types...>&, const variant<Types...>&);
      |     ^~~~~~~~~
p701.cpp:89:5: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:90:17: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   90 |   template<class... Types>
      |                 ^~~
p701.cpp:91:5: error: 'constexpr' does not name a type
   91 |     constexpr bool operator>=(const variant<Types...>&, const variant<Types...>&);
      |     ^~~~~~~~~
p701.cpp:91:5: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:92:15: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   92 | template<class... Types> requires (three_way_comparable<Types> && ...)
      |               ^~~
p701.cpp:92:35: error: expected constructor, destructor, or type conversion before '(' token
   92 | template<class... Types> requires (three_way_comparable<Types> && ...)
      |                                   ^
p701.cpp:96:30: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   96 | template<class Visitor, class... Variants>
      |                              ^~~
p701.cpp:97:1: error: 'constexpr' does not name a type
   97 | constexpr see_below visit(Visitor&&, Variants&&...);
      | ^~~~~~~~~
p701.cpp:97:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:98:39: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   98 | template<class R, class Visitor, class... Variants>
      |                                       ^~~
p701.cpp:99:5: error: 'constexpr' does not name a type
   99 |     constexpr R visit(Visitor&&, Variants&&...);
      |     ^~~~~~~~~
p701.cpp:99:5: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:102:1: error: 'constexpr' does not name a type
  102 | constexpr bool operator==(monostate, monostate) noexcept;
      | ^~~~~~~~~
p701.cpp:102:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:103:1: error: 'constexpr' does not name a type
  103 | constexpr strong_ordering operator<=>(monostate, monostate) noexcept;
      | ^~~~~~~~~
p701.cpp:103:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:105:15: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
  105 | template<class... Types>
      |               ^~~
p701.cpp:106:1: error: 'constexpr' does not name a type
  106 | constexpr void swap(variant<Types...>&, variant<Types...>&) noexcept(see_below);
      | ^~~~~~~~~
p701.cpp:106:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p701.cpp:111:15: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
  111 | template<class... Types> struct hash<variant<Types...>>;
      |               ^~~
p701.cpp:111:54: error: '>>' should be '> >' within a nested template argument list
  111 | template<class... Types> struct hash<variant<Types...>>;
      |                                                      ^~
      |                                                      > >
p701.cpp:112:24: error: 'monostate' was not declared in this scope
  112 | template<> struct hash<monostate>;
      |                        ^~~~~~~~~
p701.cpp:112:33: error: template argument 1 is invalid
  112 | template<> struct hash<monostate>;
      |                                 ^

$ g++ p701.cpp -std=2b -o p701g -I. -Wall
p701.cpp:97:11: error: 'see_below' does not name a type
   97 | constexpr see_below visit(Visitor&&, Variants&&...);
      |           ^~~~~~~~~
p701.cpp:102:16: error: declaration of 'operator==' as non-function
  102 | constexpr bool operator==(monostate, monostate) noexcept;
      |                ^~~~~~~~
p701.cpp:102:27: error: 'monostate' was not declared in this scope
  102 | constexpr bool operator==(monostate, monostate) noexcept;
      |                           ^~~~~~~~~
p701.cpp:102:38: error: 'monostate' was not declared in this scope
  102 | constexpr bool operator==(monostate, monostate) noexcept;
      |                                      ^~~~~~~~~
p701.cpp:103:27: error: declaration of 'operator<=>' as non-function
  103 | constexpr strong_ordering operator<=>(monostate, monostate) noexcept;
      |                           ^~~~~~~~
p701.cpp:103:39: error: 'monostate' was not declared in this scope
  103 | constexpr strong_ordering operator<=>(monostate, monostate) noexcept;
      |                                       ^~~~~~~~~
p701.cpp:103:50: error: 'monostate' was not declared in this scope
  103 | constexpr strong_ordering operator<=>(monostate, monostate) noexcept;
      |                                                  ^~~~~~~~~
p701.cpp:106:70: error: 'see_below' was not declared in this scope
  106 | constexpr void swap(variant<Types...>&, variant<Types...>&) noexcept(see_below);
      |                                                                      ^~~~~~~~~
p701.cpp:112:24: error: 'monostate' was not declared in this scope
  112 | template<> struct hash<monostate>;
      |                        ^~~~~~~~~
p701.cpp:112:33: error: template argument 1 is invalid
  112 | template<> struct hash<monostate>;
      |                                 ^
```

## 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)

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


typedef は C++11 ではオワコン

https://qiita.com/Linda_pp/items/44a67c64c14cba00eef1

C99からC++14を駆け抜けるC++講座

https://qiita.com/yumetodo/items/e49a673afd9a3ecb81a8


@kazuo_reve 私が効果を確認した「小川メソッド」

https://qiita.com/kazuo_reve/items/a3ea1d9171deeccc04da

## 自己参照(self reference)

ocker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 200人超えました。ありがとうございます。

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

C++N4910:2022 tag follower 200人超えました。ありがとうございます。

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

コピペコンパイルエラーあるある

https://qiita.com/kaizen_nagoya/items/29c832de9d762aed2761

C++ Error Message Collection(1)does not name a type, 11 articles

https://qiita.com/kaizen_nagoya/items/acb9f1a9b5aad6df06bd

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

Compare the contents of C++N4910:2022, C++N4741:2018 and C++N4606:2015

https://qiita.com/kaizen_nagoya/items/483246d40f98abff7ded

C++ sample list

https://qiita.com/kaizen_nagoya/items/54ad75b73825b7c04f86

clang++, g++コンパイルエラー方針の違いの例

https://qiita.com/kaizen_nagoya/items/ea6e5009fe126d270a82

astyle 使ってみた

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

C++N4606 Working Draft 2016, ISO/IEC 14882, C++ standardのコード断片をコンパイルするためにしていること
https://qiita.com/kaizen_nagoya/items/a8d7ee2f2e29e76c19c1

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

Clang/Clang++(LLVM) gcc/g++(GNU) コンパイラ警告等比較
https://qiita.com/kaizen_nagoya/items/9a82b958cc3aeef0403f

C++2003とC++2017でコンパイルエラーになるならない事例集
https://qiita.com/kaizen_nagoya/items/a13ea3823441c430edff

Qiitaに投稿するCのStyle例(暫定)
https://qiita.com/kaizen_nagoya/items/946df1528a6a1ef2bc0d

cpprefjpのdecltypeをコンパイル試験
https://qiita.com/kaizen_nagoya/items/090909af702f0d5d8a67

MISRA C++ 5-0-16
https://qiita.com/kaizen_nagoya/items/7df2d4e05db724752a74

C++ Templates Part1 BASICS Chapter 3. Class Templates 3.2 Use of Class Template Stack stack1test.cpp
https://qiita.com/kaizen_nagoya/items/cd5fc49106fad5a4e9ed

ISO/IEC TS 17961:2013 C Secure Coding Rules(1) All list(to be confirmed)
https://qiita.com/kaizen_nagoya/items/54e056195c4f11b850a1

C言語（C++)に対する誤解、曲解、無理解、爽快。
https://qiita.com/kaizen_nagoya/items/3f3992c9722c1cee2e3a

C Puzzle Bookの有り難み5つ、C言語規格及びCコンパイラの特性を認識
https://qiita.com/kaizen_nagoya/items/d89a48c1536a02ecdec9

'wchar.h' file not found で困った clang++ macOS
https://qiita.com/kaizen_nagoya/items/de15cd46d657517fac11

Open POSIX Test Suiteの使い方を調べはじめました
https://qiita.com/kaizen_nagoya/items/644d5e407f5faf96e6dc

MISRA-C 2012 Referenceに掲載している文献の入手可能性を確認
https://qiita.com/kaizen_nagoya/items/96dc8b125e462d5575bb

どうやって MISRA Example Suiteをコンパイルするか
https://qiita.com/kaizen_nagoya/items/fbdbff5ff696e2ca7f00

MISRA C まとめ #include <misra_c.h>
https://qiita.com/kaizen_nagoya/items/f1a79a7cbd281607c7c9

「C++完全理解ガイド」の同意できること上位10
https://qiita.com/kaizen_nagoya/items/aa5744e0c4a8618c7671

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 　20220726