22.5.3.2 Constructors [optional.ctor] C++N4910:2022 (547) p689.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.

# 22.5.3.2 Constructors [optional.ctor] C++N4910:2022 (547) p689.cpp

## 算譜(source code)

```c++:p689.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 = "22.5.3.2 Constructors [optional.ctor] C++N4910:2022 (547) p689.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
constexpr optional() noexcept;
constexpr optional(nullopt_t) noexcept;
// Postconditions: *this does not contain a value.
// Remarks: No contained value is initialized. For every object type T these constructors are constexpr
constructors (9.2.6).
constexpr optional(const optional& rhs);
// Effects: If rhs contains a value, direct-non-list-initializes the contained value with *rhs.
// Postconditions: rhs.has_value() == this->has_value().
// Throws: Any exception thrown by the selected constructor of T.
// Remarks: This constructor is defined as deleted unless is_copy_constructible_v<T> is true. If is_trivially_copy_constructible_v<T> is true, this constructor is trivial.
constexpr optional(optional&& rhs) noexcept(see below);
Constraints:
is_move_constructible_v<T> is true.
// Effects: If rhs contains a value, direct-non-list-initializes the contained value with std::move(*rhs). rhs.has_value() is unchanged.
// Postconditions: rhs.has_value() == this->has_value(). Throws: Any exception thrown by the selected constructor of T.
// Remarks: The exception specification is equivalent to is_nothrow_move_constructible_v<T>. If is_trivially_move_constructible_v<T> is true, this constructor is trivia
template<class... Args> constexpr explicit optional(in_place_t, Args&&... args);
// Constraints: is_constructible_v<T, Args...> is true.
// Effects: Direct-non-list-initializes the contained value with std::forward<Args>(args)....
// Postconditions: *this contains a value.
// Throws: Any exception thrown by the selected constructor of T.
// Remarks: If T’s constructor selected for the initialization is a constexpr constructor, this constructor is a constexpr constructor
template<class U, class... Args>
constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);
// Constraints: is_constructible_v<T, initializer_list<U>&, Args...> is true.
// Effects: Direct-non-list-initializes the contained value with il, std::forward<Args>(args)....
// Postconditions: *this contains a value.
// Throws: Any exception thrown by the selected constructor of T.
// Remarks: If T’s constructor selected for the initialization is a constexpr constructor, this constructor is a constexpr constructor.
template<class U = T> constexpr explicit(see_below) optional(U&& v);
// Constraints: is_constructible_v<T, U> is true, is_same_v<remove_cvref_t<U>, in_place_t> is false, and is_same_v<remove_cvref_t<U>, optional> is false.
// Effects: Direct-non-list-initializes the contained value with std::forward<U>(v).
// Postconditions: *this contains a value.
// Throws: Any exception thrown by the selected constructor of T.
// Remarks: If T’s selected constructor is a constexpr constructor, this constructor is a constexpr constructor. The expression inside explicit is equivalent to:
!is_convertible_v<U, T>
template<class U> constexpr explicit(see_below) optional(const optional<U>& rhs);
// Constraints:
// — is_constructible_v<T, const U&> is true,
// — is_constructible_v<T, optional<U>&> is false,
// — is_constructible_v<T, optional<U>&&> is false,
// — is_constructible_v<T, const optional<U>&> is false,
// — is_constructible_v<T, const optional<U>&&> is false,
// — is_convertible_v<optional<U>&, T> is false,
// — is_convertible_v<optional<U>&&, T> is false,
// — is_convertible_v<const optional<U>&, T> is false, and
// — is_convertible_v<const optional<U>&&, T> is false.
// Effects: If rhs contains a value, direct-non-list-initializes the contained value with *rhs.
// Postconditions: rhs.has_value() == this->has_value().
// Throws: Any exception thrown by the selected constructor of T.
// Remarks: The expression inside explicit is equivalent to:
!is_convertible_v<const U&, T>
template<class U> constexpr explicit(see_below) optional(optional<U>&& rhs);
// Constraints:
// — is_constructible_v<T, U> is true,
// — is_constructible_v<T, optional<U>&> is false,
// — is_constructible_v<T, optional<U>&&> is false,
// — is_constructible_v<T, const optional<U>&> is false,
// — is_constructible_v<T, const optional<U>&&> is false,
// — is_convertible_v<optional<U>&, T> is false,
// — is_convertible_v<optional<U>&&, T> is false,
// — is_convertible_v<const optional<U>&, T> is false, and
// — is_convertible_v<const optional<U>&&, T> is false.
// Effects: If rhs contains a value, direct-non-list-initializes the contained value with std::move(*rhs). rhs.has_value() is unchanged.
// Postconditions: rhs.has_value() == this->has_value().
// Throws: Any exception thrown by the selected constructor of T.
// Remarks: The expression inside explicit is equivalent to:
!is_convertible_v<U, T>
int main() {
    cout  <<  n4910 << endl;
    return EXIT_SUCCESS;
}
```

## 編纂・実行結果(compile and go)

```ubuntu:bash
$ clang++ p689.cpp -std=03 -o p689l -I. -Wall
In file included from p689.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 \
 ^
p689.cpp:27:1: error: unknown type name 'constexpr'
constexpr optional() noexcept;
^
p689.cpp:27:22: error: expected function body after function declarator
constexpr optional() noexcept;
                     ^
p689.cpp:28:1: error: unknown type name 'constexpr'
constexpr optional(nullopt_t) noexcept;
^
p689.cpp:28:20: error: unknown type name 'nullopt_t'
constexpr optional(nullopt_t) noexcept;
                   ^
p689.cpp:28:31: error: expected function body after function declarator
constexpr optional(nullopt_t) noexcept;
                              ^
p689.cpp:31:1: error: C++ requires a type specifier for all declarations
constructors (9.2.6).
^
p689.cpp:31:18: error: invalid suffix '.6' on floating constant
constructors (9.2.6).
                 ^
p689.cpp:31:21: error: expected ';' after top level declarator
constructors (9.2.6).
                    ^
                    ;
p689.cpp:37:1: error: unknown type name 'constexpr'
constexpr optional(optional&& rhs) noexcept(see below); Constraints: is_move_constructible_v<T> is true.
^
p689.cpp:37:31: error: use of undeclared identifier 'rhs'
constexpr optional(optional&& rhs) noexcept(see below); Constraints: is_move_constructible_v<T> is true.
                              ^
p689.cpp:37:35: error: expected ';' after top level declarator
constexpr optional(optional&& rhs) noexcept(see below); Constraints: is_move_constructible_v<T> is true.
                                  ^
                                  ;
p689.cpp:37:57: error: unknown type name 'Constraints'
constexpr optional(optional&& rhs) noexcept(see below); Constraints: is_move_constructible_v<T> is true.
                                                        ^
p689.cpp:37:68: error: expected unqualified-id
constexpr optional(optional&& rhs) noexcept(see below); Constraints: is_move_constructible_v<T> is true.
                                                                   ^
p689.cpp:47:24: warning: variadic templates are a C++11 extension [-Wc++11-extensions]
template<class U, class... Args>
                       ^
p689.cpp:48:3: error: unknown type name 'constexpr'
  constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);
  ^
p689.cpp:48:31: error: unknown type name 'in_place_t'
  constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);
                              ^
p689.cpp:48:43: error: no template named 'initializer_list'
  constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);
                                          ^
p689.cpp:48:71: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
  constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);
                                                                      ^
p689.cpp:54:20: error: unknown type name 'T'
template<class U = T> constexpr explicit(see_below) optional(U&& v);
                   ^
p689.cpp:54:23: error: unknown type name 'constexpr'
template<class U = T> constexpr explicit(see_below) optional(U&& v);
                      ^
p689.cpp:54:41: warning: explicit(bool) is a C++20 extension [-Wc++20-extensions]
template<class U = T> constexpr explicit(see_below) optional(U&& v);
                                        ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
3 warnings and 20 errors generated.
$ clang++ p689.cpp -std=2b -o p689l -I. -Wall
p689.cpp:27:11: error: deduction guide must be declared in the same scope as template 'std::optional'
constexpr optional() noexcept;
          ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/optional:659:11: note: template is declared here
    class optional
          ^
p689.cpp:27:11: error: deduction guide cannot be declared 'constexpr'
constexpr optional() noexcept;
~~~~~~~~~ ^
p689.cpp:27:11: error: deduction guide declaration without trailing return type
p689.cpp:28:11: error: deduction guide must be declared in the same scope as template 'std::optional'
constexpr optional(nullopt_t) noexcept;
          ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/optional:659:11: note: template is declared here
    class optional
          ^
p689.cpp:28:11: error: deduction guide cannot be declared 'constexpr'
constexpr optional(nullopt_t) noexcept;
~~~~~~~~~ ^
p689.cpp:28:11: error: deduction guide declaration without trailing return type
p689.cpp:31:1: error: no template named 'constructors'; did you mean 'construct_at'?
constructors (9.2.6).
^~~~~~~~~~~~
construct_at
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/stl_construct.h:94:5: note: 'construct_at' declared here
    construct_at(_Tp* __location, _Args&&... __args)
    ^
p689.cpp:31:1: error: C++ requires a type specifier for all declarations
constructors (9.2.6).
^
p689.cpp:31:18: error: invalid suffix '.6' on floating constant
constructors (9.2.6).
                 ^
p689.cpp:31:21: error: expected ';' after top level declarator
constructors (9.2.6).
                    ^
                    ;
p689.cpp:37:20: error: use of class template 'optional' requires template arguments; argument deduction not allowed in function prototype
constexpr optional(optional&& rhs) noexcept(see below); Constraints: is_move_constructible_v<T> is true.
                   ^~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/optional:659:11: note: template is declared here
    class optional
          ^
p689.cpp:37:45: error: use of undeclared identifier 'see'
constexpr optional(optional&& rhs) noexcept(see below); Constraints: is_move_constructible_v<T> is true.
                                            ^
p689.cpp:37:49: error: expected ')'
constexpr optional(optional&& rhs) noexcept(see below); Constraints: is_move_constructible_v<T> is true.
                                                ^
p689.cpp:37:44: note: to match this '('
constexpr optional(optional&& rhs) noexcept(see below); Constraints: is_move_constructible_v<T> is true.
                                           ^
p689.cpp:37:11: error: deduction guide must be declared in the same scope as template 'std::optional'
constexpr optional(optional&& rhs) noexcept(see below); Constraints: is_move_constructible_v<T> is true.
          ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/optional:659:11: note: template is declared here
    class optional
          ^
p689.cpp:37:11: error: deduction guide cannot be declared 'constexpr'
constexpr optional(optional&& rhs) noexcept(see below); Constraints: is_move_constructible_v<T> is true.
~~~~~~~~~ ^
p689.cpp:37:11: error: deduction guide declaration without trailing return type
p689.cpp:37:57: error: unknown type name 'Constraints'
constexpr optional(optional&& rhs) noexcept(see below); Constraints: is_move_constructible_v<T> is true.
                                                        ^
p689.cpp:37:68: error: expected unqualified-id
constexpr optional(optional&& rhs) noexcept(see below); Constraints: is_move_constructible_v<T> is true.
                                                                   ^
p689.cpp:48:22: error: deduction guide must be declared in the same scope as template 'std::optional'
  constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);
                     ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/optional:659:11: note: template is declared here
    class optional
          ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
20 errors generated.

$ g++ p689.cpp -std=03 -o p689g -I. -Wall
In file included from /usr/local/include/c++/12.1.0/atomic:38,
                 from p689.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 \
      |  ^~~~~
p689.cpp:27:1: warning: identifier 'constexpr' is a keyword in C++11 [-Wc++11-compat]
   27 | constexpr optional() noexcept;
      | ^~~~~~~~~
p689.cpp:27:22: warning: identifier 'noexcept' is a keyword in C++11 [-Wc++11-compat]
   27 | constexpr optional() noexcept;
      |                      ^~~~~~~~
p689.cpp:31:15: error: too many decimal points in number
   31 | constructors (9.2.6).
      |               ^~~~~
p689.cpp:27:1: error: 'constexpr' does not name a type
   27 | constexpr optional() noexcept;
      | ^~~~~~~~~
p689.cpp:27:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p689.cpp:28:1: error: 'constexpr' does not name a type
   28 | constexpr optional(nullopt_t) noexcept;
      | ^~~~~~~~~
p689.cpp:28:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p689.cpp:31:14: error: expected constructor, destructor, or type conversion before '(' token
   31 | constructors (9.2.6).
      |              ^
p689.cpp:37:1: error: 'constexpr' does not name a type
   37 | constexpr optional(optional&& rhs) noexcept(see below); Constraints: is_move_constructible_v<T> is true.
      | ^~~~~~~~~
p689.cpp:37:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p689.cpp:37:68: error: found ':' in nested-name-specifier, expected '::'
   37 | constexpr optional(optional&& rhs) noexcept(see below); Constraints: is_move_constructible_v<T> is true.
      |                                                                    ^
      |                                                                    ::
p689.cpp:37:57: error: 'Constraints' does not name a type
   37 | constexpr optional(optional&& rhs) noexcept(see below); Constraints: is_move_constructible_v<T> is true.
      |                                                         ^~~~~~~~~~~
p689.cpp:47:24: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   47 | template<class U, class... Args>
      |                        ^~~
p689.cpp:48:3: error: 'constexpr' does not name a type
   48 |   constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);
      |   ^~~~~~~~~
p689.cpp:48:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p689.cpp:54:20: error: 'T' does not name a type
   54 | template<class U = T> constexpr explicit(see_below) optional(U&& v);
      |                    ^
p689.cpp:54:23: error: 'constexpr' does not name a type
   54 | template<class U = T> constexpr explicit(see_below) optional(U&& v);
      |                       ^~~~~~~~~
p689.cpp:54:23: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p689.cpp:60:3: error: expected unqualified-id before '!' token
   60 |   !is_convertible_v<U, T>
      |   ^
p689.cpp:76:8: error: expected unqualified-id before '!' token
   76 |        !is_convertible_v<const U&, T>
      |        ^
p689.cpp:92:8: error: expected unqualified-id before '!' token
   92 |        !is_convertible_v<U, T>
      |        ^

$ g++ p689.cpp -std=2b -o p689g -I. -Wall
p689.cpp:31:15: error: too many decimal points in number
   31 | constructors (9.2.6).
      |               ^~~~~
p689.cpp:27:11: error: ISO C++ forbids declaration of 'optional' with no type [-fpermissive]
   27 | constexpr optional() noexcept;
      |           ^~~~~~~~
p689.cpp:28:11: error: ISO C++ forbids declaration of 'optional' with no type [-fpermissive]
   28 | constexpr optional(nullopt_t) noexcept;
      |           ^~~~~~~~
p689.cpp:28:29: error: 'constexpr const int optional' redeclared as different kind of entity
   28 | constexpr optional(nullopt_t) noexcept;
      |                             ^
p689.cpp:27:11: note: previous declaration 'constexpr int optional()'
   27 | constexpr optional() noexcept;
      |           ^~~~~~~~
p689.cpp:28:20: error: 'nullopt_t' was not declared in this scope; did you mean 'nullptr_t'?
   28 | constexpr optional(nullopt_t) noexcept;
      |                    ^~~~~~~~~
      |                    nullptr_t
p689.cpp:31:14: error: expected constructor, destructor, or type conversion before '(' token
   31 | constructors (9.2.6).
      |              ^
p689.cpp:37:11: error: ISO C++ forbids declaration of 'optional' with no type [-fpermissive]
   37 | constexpr optional(optional&& rhs) noexcept(see below); Constraints: is_move_constructible_v<T> is true.
      |           ^~~~~~~~
p689.cpp:37:20: error: 'constexpr const int optional' redeclared as different kind of entity
   37 | constexpr optional(optional&& rhs) noexcept(see below); Constraints: is_move_constructible_v<T> is true.
      |                    ^~~~~~~~
p689.cpp:27:11: note: previous declaration 'constexpr int optional()'
   27 | constexpr optional() noexcept;
      |           ^~~~~~~~
p689.cpp:37:31: error: 'rhs' was not declared in this scope
   37 | constexpr optional(optional&& rhs) noexcept(see below); Constraints: is_move_constructible_v<T> is true.
      |                               ^~~
p689.cpp:37:68: error: found ':' in nested-name-specifier, expected '::'
   37 | constexpr optional(optional&& rhs) noexcept(see below); Constraints: is_move_constructible_v<T> is true.
      |                                                                    ^
      |                                                                    ::
p689.cpp:37:57: error: 'Constraints' does not name a type
   37 | constexpr optional(optional&& rhs) noexcept(see below); Constraints: is_move_constructible_v<T> is true.
      |                                                         ^~~~~~~~~~~
p689.cpp:48:22: error: ISO C++ forbids declaration of 'optional' with no type [-fpermissive]
   48 |   constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);
      |                      ^~~~~~~~
p689.cpp:48:13: error: 'explicit' outside class declaration
   48 |   constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);
      |             ^~~~~~~~
p689.cpp:54:20: error: 'T' does not name a type
   54 | template<class U = T> constexpr explicit(see_below) optional(U&& v);
      |                    ^
p689.cpp:54:42: error: 'see_below' was not declared in this scope
   54 | template<class U = T> constexpr explicit(see_below) optional(U&& v);
      |                                          ^~~~~~~~~
p689.cpp:54:53: error: ISO C++ forbids declaration of 'optional' with no type [-fpermissive]
   54 | template<class U = T> constexpr explicit(see_below) optional(U&& v);
      |                                                     ^~~~~~~~
p689.cpp:60:3: error: expected unqualified-id before '!' token
   60 |   !is_convertible_v<U, T>
      |   ^
p689.cpp:76:8: error: expected unqualified-id before '!' token
   76 |        !is_convertible_v<const U&, T>
      |        ^
p689.cpp:92:8: error: expected unqualified-id before '!' token
   92 |        !is_convertible_v<U, 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 　20220725