22.5.2 Header <optional> synopsis [optional.syn]  C++N4910:2022 (545) p687.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.

# 22.5.2 Header <optional> synopsis [optional.syn]  C++N4910:2022 (545) p687.cpp

## 算譜(source code)

```c++:p687.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 = "22.5.2 Header <optional> synopsis [optional.syn]  C++N4910:2022 (545) p687.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 <memory>
#include <typeinfo>

using namespace std;

// Example 1
#include <compare>
namespace std {
// 22.5.3, class template optional template<class T>
class optional;
// see 17.11.1
template<class T>
constexpr bool is_optional = false;
template<class T>
constexpr bool is_optional <optional<T>> = true;
// 22.5.4, no-value state indicator
struct nullopt_t {
    see_below
};
inline constexpr nullopt_t nullopt(unspecified);
// 22.5.5, class bad_optional_access class bad_optional_access;
// exposition only // exposition only
// 22.5.6, relational operators template<class T, class U>
constexpr bool operator==(const optional<T>&, const optional<U>&);
template<class T, class U>
constexpr bool operator!=(const optional<T>&, const optional<U>&);
template<class T, class U>
constexpr bool operator<(const optional<T>&, const optional<U>&);
template<class T, class U>
constexpr bool operator>(const optional<T>&, const optional<U>&);
template<class T, class U>
constexpr bool operator<=(const optional<T>&, const optional<U>&);
template<class T, class U>
constexpr bool operator>=(const optional<T>&, const optional<U>&);
template<class T, three_way_comparable_with<T> U>
constexpr compare_three_way_result_t<T, U>
operator<=>(const optional<T>&, const optional<U>&);
// 22.5.7, comparison with nullopt
template<class T> constexpr bool operator==(const optional<T>&, nullopt_t) noexcept;
template<class T>
constexpr strong_ordering operator<=>(const optional<T>&, nullopt_t) noexcept;
// 22.5.8, comparison with T
template<class T, class U> constexpr bool operator==(const optional<T>&, const U&);
template<class T, class U> constexpr bool operator==(const T&, const optional<U>&);
template<class T, class U> constexpr bool operator!=(const optional<T>&, const U&);
template<class T, class U> constexpr bool operator!=(const T&, const optional<U>&);
template<class T, class U> constexpr bool operator<(const optional<T>&, const U&);
template<class T, class U> constexpr bool operator<(const T&, const optional<U>&);
template<class T, class U> constexpr bool operator>(const optional<T>&, const U&);
template<class T, class U> constexpr bool operator>(const T&, const optional<U>&);
template<class T, class U> constexpr bool operator<=(const optional<T>&, const U&);
template<class T, class U> constexpr bool operator<=(const T&, const optional<U>&);
template<class T, class U> constexpr bool operator>=(const optional<T>&, const U&);
template<class T, class U> constexpr bool operator>=(const T&, const optional<U>&);
template<class T, class U> requires (!is-optional<U>) && three_way_comparable_with<T, U>
constexpr compare_three_way_result_t<T, U>
operator<=>(const optional<T>&, const U&);
// 22.5.9, specialized algorithms template<class T>
constexpr void swap(optional<T>&, optional<T>&) noexcept(see_below);
template<class T>
constexpr optional<see_below> make_optional(T&&);
template<class T, class... Args>
constexpr optional<T> make_optional(Args&&... args);
template<class T, class U, class... Args>
constexpr optional<T> make_optional(initializer_list<U> il, Args&&... args);
// 22.5.10, hash support
template<class T> struct hash;
template<class T> struct hash<optional<T>>;
}
int main() {
    cout  <<  n4910 << endl;
    return EXIT_SUCCESS;
}
```

## 編纂・実行結果(compile and go)

```ubuntu:bash
$ clang++ p687.cpp -std=03 -o p687l -I. -Wall
In file included from p687.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 \
 ^
p687.cpp:33:1: error: unknown type name 'constexpr'
constexpr bool is_optional = false;
^
p687.cpp:33:16: warning: variable templates are a C++14 extension [-Wc++14-extensions]
constexpr bool is_optional = false;
               ^
p687.cpp:35:1: error: unknown type name 'constexpr'
constexpr bool is_optional <optional<T>> = true;
^
p687.cpp:35:37: error: expected '>'
constexpr bool is_optional <optional<T>> = true;
                                    ^
p687.cpp:35:28: note: to match this '<'
constexpr bool is_optional <optional<T>> = true;
                           ^
p687.cpp:35:37: error: expected unqualified-id
constexpr bool is_optional <optional<T>> = true;
                                    ^
p687.cpp:37:18: error: unknown type name 'see_below'
struct nullopt_t{see_below};
                 ^
p687.cpp:37:27: error: expected member name or ';' after declaration specifiers
struct nullopt_t{see_below};
                 ~~~~~~~~~^
p687.cpp:38:8: error: unknown type name 'constexpr'
inline constexpr nullopt_t nullopt(unspecified);
       ^
p687.cpp:38:1: warning: inline variables are a C++17 extension [-Wc++17-extensions]
inline constexpr nullopt_t nullopt(unspecified);
^
p687.cpp:38:27: error: expected ';' after top level declarator
inline constexpr nullopt_t nullopt(unspecified);
                          ^
                          ;
p687.cpp:42:5: error: unknown type name 'constexpr'
    constexpr bool operator==(const optional<T>&, const optional<U>&);
    ^
p687.cpp:42:45: error: expected ')'
    constexpr bool operator==(const optional<T>&, const optional<U>&);
                                            ^
p687.cpp:42:30: note: to match this '('
    constexpr bool operator==(const optional<T>&, const optional<U>&);
                             ^
p687.cpp:44:5: error: unknown type name 'constexpr'
    constexpr bool operator!=(const optional<T>&, const optional<U>&);
    ^
p687.cpp:44:45: error: expected ')'
    constexpr bool operator!=(const optional<T>&, const optional<U>&);
                                            ^
p687.cpp:44:30: note: to match this '('
    constexpr bool operator!=(const optional<T>&, const optional<U>&);
                             ^
p687.cpp:46:5: error: unknown type name 'constexpr'
    constexpr bool operator<(const optional<T>&, const optional<U>&);
    ^
p687.cpp:46:44: error: expected ')'
    constexpr bool operator<(const optional<T>&, const optional<U>&);
                                           ^
p687.cpp:46:29: note: to match this '('
    constexpr bool operator<(const optional<T>&, const optional<U>&);
                            ^
p687.cpp:48:5: error: unknown type name 'constexpr'
    constexpr bool operator>(const optional<T>&, const optional<U>&);
    ^
p687.cpp:48:44: error: expected ')'
    constexpr bool operator>(const optional<T>&, const optional<U>&);
                                           ^
p687.cpp:48:29: note: to match this '('
    constexpr bool operator>(const optional<T>&, const optional<U>&);
                            ^
p687.cpp:50:5: error: unknown type name 'constexpr'
    constexpr bool operator<=(const optional<T>&, const optional<U>&);
    ^
p687.cpp:50:45: error: expected ')'
    constexpr bool operator<=(const optional<T>&, const optional<U>&);
                                            ^
p687.cpp:50:30: note: to match this '('
    constexpr bool operator<=(const optional<T>&, const optional<U>&);
                             ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
2 warnings and 20 errors generated.
$ clang++ p687.cpp -std=2b -o p687l -I. -Wall
p687.cpp:30:11: error: redefinition of 'optional' as different kind of symbol
    class optional;
          ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/optional:659:11: note: previous definition is here
    class optional
          ^
p687.cpp:37:8: error: redefinition of 'nullopt_t'
struct nullopt_t{see_below};
       ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/optional:63:10: note: previous definition is here
  struct nullopt_t
         ^
p687.cpp:38:36: error: unknown type name 'unspecified'
inline constexpr nullopt_t nullopt(unspecified);
                                   ^
p687.cpp:42:46: error: use of undeclared identifier 'T'
    constexpr bool operator==(const optional<T>&, const optional<U>&);
                                             ^
p687.cpp:42:66: error: use of undeclared identifier 'U'
    constexpr bool operator==(const optional<T>&, const optional<U>&);
                                                                 ^
p687.cpp:73:53: error: expected '(' for function-style cast or type construction
template<class T, class U> requires (!is-optional<U>) && three_way_comparable_with<T, U>
                                         ~~~~~~~~~~~^
p687.cpp:73:39: error: use of undeclared identifier 'is'
template<class T, class U> requires (!is-optional<U>) && three_way_comparable_with<T, U>
                                      ^
p687.cpp:77:30: error: use of undeclared identifier 'T'
constexpr void swap(optional<T>&, optional<T>&) noexcept(see_below);
                             ^
p687.cpp:77:44: error: use of undeclared identifier 'T'
constexpr void swap(optional<T>&, optional<T>&) noexcept(see_below);
                                           ^
p687.cpp:77:58: error: use of undeclared identifier 'see_below'
constexpr void swap(optional<T>&, optional<T>&) noexcept(see_below);
                                                         ^
p687.cpp:79:20: error: use of undeclared identifier 'see_below'
constexpr optional<see_below> make_optional(T&&);
                   ^
11 errors generated.

$ g++ p687.cpp -std=03 -o p687g -I. -Wall
In file included from /usr/local/include/c++/12.1.0/atomic:38,
                 from p687.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 \
      |  ^~~~~
p687.cpp:33:1: warning: identifier 'constexpr' is a keyword in C++11 [-Wc++11-compat]
   33 | constexpr bool is_optional = false;
      | ^~~~~~~~~
p687.cpp:57:76: warning: identifier 'noexcept' is a keyword in C++11 [-Wc++11-compat]
   57 | template<class T> constexpr bool operator==(const optional<T>&, nullopt_t) noexcept;
      |                                                                            ^~~~~~~~
p687.cpp:33:1: error: 'constexpr' does not name a type
   33 | constexpr bool is_optional = false;
      | ^~~~~~~~~
p687.cpp:33:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:35:1: error: 'constexpr' does not name a type
   35 | constexpr bool is_optional <optional<T>> = true;
      | ^~~~~~~~~
p687.cpp:35:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:37:18: error: 'see_below' does not name a type
   37 | struct nullopt_t{see_below};
      |                  ^~~~~~~~~
p687.cpp:38:8: error: 'constexpr' does not name a type
   38 | inline constexpr nullopt_t nullopt(unspecified);
      |        ^~~~~~~~~
p687.cpp:38:8: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:42:5: error: 'constexpr' does not name a type
   42 |     constexpr bool operator==(const optional<T>&, const optional<U>&);
      |     ^~~~~~~~~
p687.cpp:42:5: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:44:5: error: 'constexpr' does not name a type
   44 |     constexpr bool operator!=(const optional<T>&, const optional<U>&);
      |     ^~~~~~~~~
p687.cpp:44:5: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:46:5: error: 'constexpr' does not name a type
   46 |     constexpr bool operator<(const optional<T>&, const optional<U>&);
      |     ^~~~~~~~~
p687.cpp:46:5: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:48:5: error: 'constexpr' does not name a type
   48 |     constexpr bool operator>(const optional<T>&, const optional<U>&);
      |     ^~~~~~~~~
p687.cpp:48:5: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:50:5: error: 'constexpr' does not name a type
   50 |     constexpr bool operator<=(const optional<T>&, const optional<U>&);
      |     ^~~~~~~~~
p687.cpp:50:5: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:52:5: error: 'constexpr' does not name a type
   52 |     constexpr bool operator>=(const optional<T>&, const optional<U>&);
      |     ^~~~~~~~~
p687.cpp:52:5: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:53:21: error: 'three_way_comparable_with' has not been declared
   53 |   template<class T, three_way_comparable_with<T> U>
      |                     ^~~~~~~~~~~~~~~~~~~~~~~~~
p687.cpp:53:46: error: expected '>' before '<' token
   53 |   template<class T, three_way_comparable_with<T> U>
      |                                              ^
p687.cpp:54:5: error: 'constexpr' does not name a type
   54 |     constexpr compare_three_way_result_t<T, U>
      |     ^~~~~~~~~
p687.cpp:54:5: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:57:19: error: 'constexpr' does not name a type
   57 | template<class T> constexpr bool operator==(const optional<T>&, nullopt_t) noexcept;
      |                   ^~~~~~~~~
p687.cpp:57:19: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:59:5: error: 'constexpr' does not name a type
   59 |     constexpr strong_ordering operator<=>(const optional<T>&, nullopt_t) noexcept;
      |     ^~~~~~~~~
p687.cpp:59:5: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:61:28: error: 'constexpr' does not name a type
   61 | template<class T, class U> constexpr bool operator==(const optional<T>&, const U&);
      |                            ^~~~~~~~~
p687.cpp:61:28: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:62:28: error: 'constexpr' does not name a type
   62 | template<class T, class U> constexpr bool operator==(const T&, const optional<U>&);
      |                            ^~~~~~~~~
p687.cpp:62:28: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:63:28: error: 'constexpr' does not name a type
   63 | template<class T, class U> constexpr bool operator!=(const optional<T>&, const U&);
      |                            ^~~~~~~~~
p687.cpp:63:28: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:64:28: error: 'constexpr' does not name a type
   64 | template<class T, class U> constexpr bool operator!=(const T&, const optional<U>&);
      |                            ^~~~~~~~~
p687.cpp:64:28: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:65:28: error: 'constexpr' does not name a type
   65 | template<class T, class U> constexpr bool operator<(const optional<T>&, const U&);
      |                            ^~~~~~~~~
p687.cpp:65:28: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:66:28: error: 'constexpr' does not name a type
   66 | template<class T, class U> constexpr bool operator<(const T&, const optional<U>&);
      |                            ^~~~~~~~~
p687.cpp:66:28: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:67:28: error: 'constexpr' does not name a type
   67 | template<class T, class U> constexpr bool operator>(const optional<T>&, const U&);
      |                            ^~~~~~~~~
p687.cpp:67:28: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:68:28: error: 'constexpr' does not name a type
   68 | template<class T, class U> constexpr bool operator>(const T&, const optional<U>&);
      |                            ^~~~~~~~~
p687.cpp:68:28: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:69:28: error: 'constexpr' does not name a type
   69 | template<class T, class U> constexpr bool operator<=(const optional<T>&, const U&);
      |                            ^~~~~~~~~
p687.cpp:69:28: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:70:28: error: 'constexpr' does not name a type
   70 | template<class T, class U> constexpr bool operator<=(const T&, const optional<U>&);
      |                            ^~~~~~~~~
p687.cpp:70:28: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:71:28: error: 'constexpr' does not name a type
   71 | template<class T, class U> constexpr bool operator>=(const optional<T>&, const U&);
      |                            ^~~~~~~~~
p687.cpp:71:28: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:72:28: error: 'constexpr' does not name a type
   72 | template<class T, class U> constexpr bool operator>=(const T&, const optional<U>&);
      |                            ^~~~~~~~~
p687.cpp:72:28: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:73:37: error: expected constructor, destructor, or type conversion before '(' token
   73 | template<class T, class U> requires (!is-optional<U>) && three_way_comparable_with<T, U>
      |                                     ^
p687.cpp:77:1: error: 'constexpr' does not name a type
   77 | constexpr void swap(optional<T>&, optional<T>&) noexcept(see_below);
      | ^~~~~~~~~
p687.cpp:77:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:79:1: error: 'constexpr' does not name a type
   79 | constexpr optional<see_below> make_optional(T&&);
      | ^~~~~~~~~
p687.cpp:79:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:80:26: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   80 |   template<class T, class... Args>
      |                          ^~~
p687.cpp:81:5: error: 'constexpr' does not name a type
   81 |     constexpr optional<T> make_optional(Args&&... args);
      |     ^~~~~~~~~
p687.cpp:81:5: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:82:35: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   82 |   template<class T, class U, class... Args>
      |                                   ^~~
p687.cpp:83:5: error: 'constexpr' does not name a type
   83 |     constexpr optional<T> make_optional(initializer_list<U> il, Args&&... args);
      |     ^~~~~~~~~
p687.cpp:83:5: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p687.cpp:85:62: error: 'std::optional' is not a template
   85 | template<class T> struct hash; template<class T> struct hash<optional<T>>;
      |                                                              ^~~~~~~~
p687.cpp:85:72: error: '>>' should be '> >' within a nested template argument list
   85 | template<class T> struct hash; template<class T> struct hash<optional<T>>;
      |                                                                        ^~
      |                                                                        > >
p687.cpp:85:57: error: template parameters not deducible in partial specialization:
   85 | template<class T> struct hash; template<class T> struct hash<optional<T>>;
      |                                                         ^~~~~~~~~~~~~~~~~
p687.cpp:85:57: note:         'T'

$ g++ p687.cpp -std=2b -o p687g -I. -Wall
p687.cpp:35:29: error: 'std::optional' is not a template
   35 | constexpr bool is_optional <optional<T>> = true;
      |                             ^~~~~~~~
p687.cpp:35:16: error: template-id 'is_optional<std::optional>' in declaration of primary template
   35 | constexpr bool is_optional <optional<T>> = true;
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~
p687.cpp:35:16: error: redefinition of 'template<class T> constexpr const bool std::is_optional'
p687.cpp:33:16: note: 'template<class T> constexpr const bool std::is_optional<T>' previously declared here
   33 | constexpr bool is_optional = false;
      |                ^~~~~~~~~~~
p687.cpp:37:18: error: 'see_below' does not name a type
   37 | struct nullopt_t{see_below};
      |                  ^~~~~~~~~
p687.cpp:38:36: error: 'unspecified' was not declared in this scope
   38 | inline constexpr nullopt_t nullopt(unspecified);
      |                                    ^~~~~~~~~~~
p687.cpp:42:37: error: 'std::optional' is not a template
   42 |     constexpr bool operator==(const optional<T>&, const optional<U>&);
      |                                     ^~~~~~~~
p687.cpp:42:46: error: 'T' was not declared in this scope
   42 |     constexpr bool operator==(const optional<T>&, const optional<U>&);
      |                                              ^
p687.cpp:42:57: error: 'std::optional' is not a template
   42 |     constexpr bool operator==(const optional<T>&, const optional<U>&);
      |                                                         ^~~~~~~~
p687.cpp:42:66: error: 'U' was not declared in this scope
   42 |     constexpr bool operator==(const optional<T>&, const optional<U>&);
      |                                                                  ^
p687.cpp:44:37: error: 'std::optional' is not a template
   44 |     constexpr bool operator!=(const optional<T>&, const optional<U>&);
      |                                     ^~~~~~~~
p687.cpp:44:57: error: 'std::optional' is not a template
   44 |     constexpr bool operator!=(const optional<T>&, const optional<U>&);
      |                                                         ^~~~~~~~
p687.cpp:46:36: error: 'std::optional' is not a template
   46 |     constexpr bool operator<(const optional<T>&, const optional<U>&);
      |                                    ^~~~~~~~
p687.cpp:46:56: error: 'std::optional' is not a template
   46 |     constexpr bool operator<(const optional<T>&, const optional<U>&);
      |                                                        ^~~~~~~~
p687.cpp:48:36: error: 'std::optional' is not a template
   48 |     constexpr bool operator>(const optional<T>&, const optional<U>&);
      |                                    ^~~~~~~~
p687.cpp:48:56: error: 'std::optional' is not a template
   48 |     constexpr bool operator>(const optional<T>&, const optional<U>&);
      |                                                        ^~~~~~~~
p687.cpp:50:37: error: 'std::optional' is not a template
   50 |     constexpr bool operator<=(const optional<T>&, const optional<U>&);
      |                                     ^~~~~~~~
p687.cpp:50:57: error: 'std::optional' is not a template
   50 |     constexpr bool operator<=(const optional<T>&, const optional<U>&);
      |                                                         ^~~~~~~~
p687.cpp:52:37: error: 'std::optional' is not a template
   52 |     constexpr bool operator>=(const optional<T>&, const optional<U>&);
      |                                     ^~~~~~~~
p687.cpp:52:57: error: 'std::optional' is not a template
   52 |     constexpr bool operator>=(const optional<T>&, const optional<U>&);
      |                                                         ^~~~~~~~
p687.cpp:55:25: error: 'std::optional' is not a template
   55 |       operator<=>(const optional<T>&, const optional<U>&);
      |                         ^~~~~~~~
p687.cpp:55:45: error: 'std::optional' is not a template
   55 |       operator<=>(const optional<T>&, const optional<U>&);
      |                                             ^~~~~~~~
p687.cpp:57:51: error: 'std::optional' is not a template
   57 | template<class T> constexpr bool operator==(const optional<T>&, nullopt_t) noexcept;
      |                                                   ^~~~~~~~
p687.cpp:59:49: error: 'std::optional' is not a template
   59 |     constexpr strong_ordering operator<=>(const optional<T>&, nullopt_t) noexcept;
      |                                                 ^~~~~~~~
p687.cpp:61:60: error: 'std::optional' is not a template
   61 | template<class T, class U> constexpr bool operator==(const optional<T>&, const U&);
      |                                                            ^~~~~~~~
p687.cpp:62:70: error: 'std::optional' is not a template
   62 | template<class T, class U> constexpr bool operator==(const T&, const optional<U>&);
      |                                                                      ^~~~~~~~
p687.cpp:63:60: error: 'std::optional' is not a template
   63 | template<class T, class U> constexpr bool operator!=(const optional<T>&, const U&);
      |                                                            ^~~~~~~~
p687.cpp:64:70: error: 'std::optional' is not a template
   64 | template<class T, class U> constexpr bool operator!=(const T&, const optional<U>&);
      |                                                                      ^~~~~~~~
p687.cpp:65:59: error: 'std::optional' is not a template
   65 | template<class T, class U> constexpr bool operator<(const optional<T>&, const U&);
      |                                                           ^~~~~~~~
p687.cpp:66:69: error: 'std::optional' is not a template
   66 | template<class T, class U> constexpr bool operator<(const T&, const optional<U>&);
      |                                                                     ^~~~~~~~
p687.cpp:67:59: error: 'std::optional' is not a template
   67 | template<class T, class U> constexpr bool operator>(const optional<T>&, const U&);
      |                                                           ^~~~~~~~
p687.cpp:68:69: error: 'std::optional' is not a template
   68 | template<class T, class U> constexpr bool operator>(const T&, const optional<U>&);
      |                                                                     ^~~~~~~~
p687.cpp:69:60: error: 'std::optional' is not a template
   69 | template<class T, class U> constexpr bool operator<=(const optional<T>&, const U&);
      |                                                            ^~~~~~~~
p687.cpp:70:70: error: 'std::optional' is not a template
   70 | template<class T, class U> constexpr bool operator<=(const T&, const optional<U>&);
      |                                                                      ^~~~~~~~
p687.cpp:71:60: error: 'std::optional' is not a template
   71 | template<class T, class U> constexpr bool operator>=(const optional<T>&, const U&);
      |                                                            ^~~~~~~~
p687.cpp:72:70: error: 'std::optional' is not a template
   72 | template<class T, class U> constexpr bool operator>=(const T&, const optional<U>&);
      |                                                                      ^~~~~~~~
p687.cpp:73:39: error: 'is' was not declared in this scope; did you mean 'ws'?
   73 | template<class T, class U> requires (!is-optional<U>) && three_way_comparable_with<T, U>
      |                                       ^~
      |                                       ws
p687.cpp:73:42: error: 'std::optional' is not a template
   73 | template<class T, class U> requires (!is-optional<U>) && three_way_comparable_with<T, U>
      |                                          ^~~~~~~~
p687.cpp:73:53: error: expected primary-expression before ')' token
   73 | template<class T, class U> requires (!is-optional<U>) && three_way_comparable_with<T, U>
      |                                                     ^
p687.cpp:75:25: error: 'std::optional' is not a template
   75 |       operator<=>(const optional<T>&, const U&);
      |                         ^~~~~~~~
p687.cpp:77:21: error: 'std::optional' is not a template
   77 | constexpr void swap(optional<T>&, optional<T>&) noexcept(see_below);
      |                     ^~~~~~~~
p687.cpp:77:30: error: 'T' was not declared in this scope
   77 | constexpr void swap(optional<T>&, optional<T>&) noexcept(see_below);
      |                              ^
p687.cpp:77:35: error: 'std::optional' is not a template
   77 | constexpr void swap(optional<T>&, optional<T>&) noexcept(see_below);
      |                                   ^~~~~~~~
p687.cpp:77:44: error: 'T' was not declared in this scope
   77 | constexpr void swap(optional<T>&, optional<T>&) noexcept(see_below);
      |                                            ^
p687.cpp:77:58: error: 'see_below' was not declared in this scope
   77 | constexpr void swap(optional<T>&, optional<T>&) noexcept(see_below);
      |                                                          ^~~~~~~~~
p687.cpp:79:11: error: 'std::optional' is not a template
   79 | constexpr optional<see_below> make_optional(T&&);
      |           ^~~~~~~~
p687.cpp:79:20: error: 'see_below' was not declared in this scope
   79 | constexpr optional<see_below> make_optional(T&&);
      |                    ^~~~~~~~~
p687.cpp:81:15: error: 'std::optional' is not a template
   81 |     constexpr optional<T> make_optional(Args&&... args);
      |               ^~~~~~~~
p687.cpp:83:15: error: 'std::optional' is not a template
   83 |     constexpr optional<T> make_optional(initializer_list<U> il, Args&&... args);
      |               ^~~~~~~~
p687.cpp:85:62: error: 'std::optional' is not a template
   85 | template<class T> struct hash; template<class T> struct hash<optional<T>>;
      |                                                              ^~~~~~~~
p687.cpp:85:57: error: template parameters not deducible in partial specialization:
   85 | template<class T> struct hash; template<class T> struct hash<optional<T>>;
      |                                                         ^~~~~~~~~~~~~~~~~
p687.cpp:85:57: note:         'T'
```

## 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 　20220724