22.8.6.2 Constructors [expected.object.ctor] C++N4910:2022 (594) p721.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.

# 22.8.6.2 Constructors [expected.object.ctor] C++N4910:2022 (594) p721.cpp

## 算譜(source code)

```c++:p721.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 = "22.8.6.2 Constructors [expected.object.ctor] C++N4910:2022 (594) p721.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 <variant>
#include <any>
#include <typeinfo>

using namespace std;

// Example 1
constexpr expected();
// Constraints: is_default_constructible_v<T> is true.
// Effects: Value-initializes val .
// Postconditions: has_value() is true.
// Throws: Any exception thrown by the initialization of val .
constexpr expected(const expected& rhs);
// Effects: If rhs.has_value() is true, direct-non-list-initializes val with *rhs. Otherwise, direct-non- list-initializes unex with rhs.error().
// Postconditions: rhs.has_value() == this->has_value().
// Throws: Any exception thrown by the initialization of val or unex .
// Remarks: This constructor is defined as deleted unless
// — is_copy_constructible_v<T> is true and
// — is_copy_constructible_v<E> is true. This constructor is trivial if
// — is_trivially_copy_constructible_v<T> is true and
// — is_trivially_copy_constructible_v<E> is true.
constexpr expected(expected&& rhs) noexcept(see_below);
// Constraints: — is_move_constructible_v<T> is true and
// — is_move_constructible_v<E> is true.
// Effects: If rhs.has_value() is true, direct-non-list-initializes val with std::move(*rhs). Otherwise, direct-non-list-initializes unex with std::move(rhs.error()).
// Postconditions: rhs.has_value() is unchanged; rhs.has_value() == this->has_value() is true.
// Throws: Any exception thrown by the initialization of val or unex .
// Remarks: The exception specification is equivalent to is_nothrow_move_constructible_v<T> && is_nothrow_move_constructible_v<E>.
// This constructor is trivial if
// — is_trivially_move_constructible_v<T> is true and
// — is_trivially_move_constructible_v<E> is true.
template<class U, class G>
constexpr explicit(see_below) expected(const expected<U, G>& rhs);
template<class U, class G>
constexpr explicit(see_below) expected(expected<U, G>&& rhs);
// Let: — UF be const U& for the first overload and U for the second overload.
// — GF be const G& for the first overload and G for the second overload.
// Constraints: - is_constructible_v<T, UF> is true; and
// - is_constructible_v<E, GF> is true; and
// — is_constructible_v<T, expected<U, G>&> is false; and
// — is_constructible_v<T, expected<U, G>> is false; and
// — is_constructible_v<T, const expected<U, G>&> is false; and
// — is_constructible_v<T, const expected<U, G>> is false; and
// — is_convertible_v<expected<U, G>&, T> is false; and
// — is_convertible_v<expected<U, G>&&, T> is false; and
// — is_convertible_v<const expected<U, G>&, T> is false; and
// — is_convertible_v<const expected<U, G>&&, T> is false; and
// — is_constructible_v<unexpected<E>, expected<U, G>&> is false; and
// — is_constructible_v<unexpected<E>, expected<U, G>> is false; and
// — is_constructible_v<unexpected<E>, const expected<U, G>&> is false; and
// — is_constructible_v<unexpected<E>, const expected<U, G>> is false.
// Effects: If rhs.has_value(), direct-non-list-initializes val with std::forward<UF>(*rhs). Otherwise, direct-non-list-initializes unex with std::forward<GF>(rhs.error()).
// Postconditions: rhs.has_value() is unchanged; rhs.has_value() == this->has_value() is true.
// Throws: Any exception thrown by the initialization of val or unex .
// Remarks: The expression inside explicit is equivalent to !is_convertible_v<UF, T> || !is_- convertible_v<GF, E>.
template<class U = T>
constexpr explicit(!is_convertible_v<U, T>) expected(U&& v);
// Constraints:— is_same_v<remove_cvref_t<U>, in_place_t> is false; and
// — is_same_v<expected<T, E>, remove_cvref_t<U>> is false; and
// — remove_cvref_t<U> is not a specialization of unexpected; and
// — is_constructible_v<T, U> is true.
// Effects: Direct-non-list-initializes val with std::forward<U>(v).
// Postconditions: has_value() is true.
// Throws: Any exception thrown by the initialization of val .
template<class G>
constexpr explicit(!is_convertible_v<const G&, E>) expected(const unexpected<G>& e);
template<class G>
constexpr explicit(!is_convertible_v<G, E>) expected(unexpected<G>&& e);
// Let GF be const G& for the first overload and G for the second overload.
// Constraints: is_constructible_v<E, GF> is true.
// Effects: Direct-non-list-initializes unex with std::forward<GF>(e.value()).
// Postconditions: has_value() is false.
// Throws: Any exception thrown by the initialization of unex .
template<class... Args>
constexpr explicit expected(in_place_t, Args&&... args);
// Constraints: is_constructible_v<T, Args...> is true.
// Effects: Direct-non-list-initializes val with std::forward<Args>(args)....
// Postconditions: has_value() is true.
// Throws: Any exception thrown by the initialization of val .
template<class U, class... Args>
constexpr explicit expected(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 val with il, std::forward<Args>(args)....
// Postconditions: has_value() is true.
// Throws: Any exception thrown by the initialization of val .subclause makes use of the following exposition-only function:
template<class T, class U, class... Args>
constexpr void reinit-expected (T& newval, U& oldval, Args&&... args) {
    if constexpr (is_nothrow_constructible_v<T, Args...>) {
        destroy_at(addressof(oldval));
        construct_at(addressof(newval), std::forward<Args>(args)...);
    } else if constexpr (is_nothrow_move_constructible_v<T>) {
        T tmp(std::forward<Args>(args)...);
        destroy_at(addressof(oldval));
        construct_at(addressof(newval), std::move(tmp));
    } else {
        U tmp(std::move(oldval));
        destroy_at(addressof(oldval));
        try {
            construct_at(addressof(newval), std::forward<Args>(args)...);
        } catch (...) {
            construct_at(addressof(oldval), std::move(tmp));
            throw;
        }
    }
}
constexpr expected& operator=(const expected& rhs);
template<class... Args>
constexpr explicit expected(unexpect_t, Args&&... args);
// Constraints: is_constructible_v<E, Args...> is true.
// Effects: Direct-non-list-initializes unex with std::forward<Args>(args)....
// Postconditions: has_value() is false.
// Throws: Any exception thrown by the initialization of unex .
template<class U, class... Args>
constexpr explicit expected(unexpect_t, initializer_list<U> il, Args&&... args);
// Constraints: is_constructible_v<E, initializer_list<U>&, Args...> is true.
// Effects: Direct-non-list-initializes unex with il, std::forward<Args>(args)....
// Postconditions: has_value() is false.
// Throws: Any exception thrown by the initialization of unex .
int main() {
    cout  <<  n4910 << endl;
    return EXIT_SUCCESS;
}
```

## 編纂・実行結果(compile and go)

```ubuntu:bash
$ clang++ p721.cpp -std=03 -o p721l -I. -Wall
In file included from p721.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 \
 ^
p721.cpp:30:1: error: unknown type name 'constexpr'
constexpr expected();
^
p721.cpp:35:1: error: unknown type name 'constexpr'
constexpr expected(const expected& rhs);
^
p721.cpp:35:26: error: unknown type name 'expected'
constexpr expected(const expected& rhs);
                         ^
p721.cpp:44:1: error: unknown type name 'constexpr'
constexpr expected(expected&& rhs) noexcept(see_below); 
^
p721.cpp:44:31: error: use of undeclared identifier 'rhs'
constexpr expected(expected&& rhs) noexcept(see_below); 
                              ^
p721.cpp:44:35: error: expected ';' after top level declarator
constexpr expected(expected&& rhs) noexcept(see_below); 
                                  ^
                                  ;
p721.cpp:55:1: error: unknown type name 'constexpr'
constexpr explicit(see_below) expected(const expected<U, G>& rhs);
^
p721.cpp:55:19: warning: explicit(bool) is a C++20 extension [-Wc++20-extensions]
constexpr explicit(see_below) expected(const expected<U, G>& rhs);
                  ^
p721.cpp:55:20: error: use of undeclared identifier 'see_below'
constexpr explicit(see_below) expected(const expected<U, G>& rhs);
                   ^
p721.cpp:55:46: error: no template named 'expected'
constexpr explicit(see_below) expected(const expected<U, G>& rhs);
                                             ^
p721.cpp:57:1: error: unknown type name 'constexpr'
constexpr explicit(see_below) expected(expected<U, G>&& rhs);
^
p721.cpp:57:19: warning: explicit(bool) is a C++20 extension [-Wc++20-extensions]
constexpr explicit(see_below) expected(expected<U, G>&& rhs);
                  ^
p721.cpp:57:20: error: use of undeclared identifier 'see_below'
constexpr explicit(see_below) expected(expected<U, G>&& rhs);
                   ^
p721.cpp:57:11: error: 'explicit' can only appear on non-static member functions
constexpr explicit(see_below) expected(expected<U, G>&& rhs);
          ^
p721.cpp:57:31: warning: variable templates are a C++14 extension [-Wc++14-extensions]
constexpr explicit(see_below) expected(expected<U, G>&& rhs);
                              ^
p721.cpp:57:57: error: use of undeclared identifier 'rhs'
constexpr explicit(see_below) expected(expected<U, G>&& rhs);
                                                        ^
p721.cpp:78:20: error: unknown type name 'T'
template<class U = T>
                   ^
p721.cpp:79:3: error: unknown type name 'constexpr'
  constexpr explicit(!is_convertible_v<U, T>) expected(U&& v);
  ^
p721.cpp:79:21: warning: explicit(bool) is a C++20 extension [-Wc++20-extensions]
  constexpr explicit(!is_convertible_v<U, T>) expected(U&& v);
                    ^
p721.cpp:79:43: error: use of undeclared identifier 'T'
  constexpr explicit(!is_convertible_v<U, T>) expected(U&& v);
                                          ^
p721.cpp:79:57: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
  constexpr explicit(!is_convertible_v<U, T>) expected(U&& v);
                                                        ^
p721.cpp:88:3: error: unknown type name 'constexpr'
  constexpr explicit(!is_convertible_v<const G&, E>) expected(const unexpected<G>& e);
  ^
p721.cpp:88:21: warning: explicit(bool) is a C++20 extension [-Wc++20-extensions]
  constexpr explicit(!is_convertible_v<const G&, E>) expected(const unexpected<G>& e);
                    ^
p721.cpp:88:50: error: use of undeclared identifier 'E'
  constexpr explicit(!is_convertible_v<const G&, E>) expected(const unexpected<G>& e);
                                                 ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
6 warnings and 20 errors generated.
$ clang++ p721.cpp -std=2b -o p721l -I. -Wall
p721.cpp:30:11: error: C++ requires a type specifier for all declarations
constexpr expected();
~~~~~~~~~ ^
p721.cpp:35:26: error: unknown type name 'expected'
constexpr expected(const expected& rhs);
                         ^
p721.cpp:35:11: error: C++ requires a type specifier for all declarations
constexpr expected(const expected& rhs);
~~~~~~~~~ ^
p721.cpp:44:11: error: C++ requires a type specifier for all declarations
constexpr expected(expected&& rhs) noexcept(see_below); 
~~~~~~~~~ ^
p721.cpp:44:31: error: use of undeclared identifier 'rhs'
constexpr expected(expected&& rhs) noexcept(see_below); 
                              ^
p721.cpp:44:35: error: expected ';' after top level declarator
constexpr expected(expected&& rhs) noexcept(see_below); 
                                  ^
                                  ;
p721.cpp:55:20: error: use of undeclared identifier 'see_below'
constexpr explicit(see_below) expected(const expected<U, G>& rhs);
                   ^
p721.cpp:55:46: error: parameter name cannot have template arguments
constexpr explicit(see_below) expected(const expected<U, G>& rhs);
                                             ^
p721.cpp:55:46: error: C++ requires a type specifier for all declarations
constexpr explicit(see_below) expected(const expected<U, G>& rhs);
                                       ~~~~~ ^
p721.cpp:55:60: error: expected ')'
constexpr explicit(see_below) expected(const expected<U, G>& rhs);
                                                           ^
p721.cpp:55:39: note: to match this '('
constexpr explicit(see_below) expected(const expected<U, G>& rhs);
                                      ^
p721.cpp:55:31: error: C++ requires a type specifier for all declarations
constexpr explicit(see_below) expected(const expected<U, G>& rhs);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^
p721.cpp:57:20: error: use of undeclared identifier 'see_below'
constexpr explicit(see_below) expected(expected<U, G>&& rhs);
                   ^
p721.cpp:57:31: error: C++ requires a type specifier for all declarations
constexpr explicit(see_below) expected(expected<U, G>&& rhs);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^
p721.cpp:57:11: error: 'explicit' can only appear on non-static member functions
constexpr explicit(see_below) expected(expected<U, G>&& rhs);
          ^
p721.cpp:57:57: error: use of undeclared identifier 'rhs'
constexpr explicit(see_below) expected(expected<U, G>&& rhs);
                                                        ^
p721.cpp:78:20: error: unknown type name 'T'
template<class U = T>
                   ^
p721.cpp:79:43: error: use of undeclared identifier 'T'
  constexpr explicit(!is_convertible_v<U, T>) expected(U&& v);
                                          ^
p721.cpp:79:47: error: C++ requires a type specifier for all declarations
  constexpr explicit(!is_convertible_v<U, T>) expected(U&& v);
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^
p721.cpp:88:50: error: use of undeclared identifier 'E'
  constexpr explicit(!is_convertible_v<const G&, E>) expected(const unexpected<G>& e);
                                                 ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
20 errors generated.

$ g++ p721.cpp -std=03 -o p721g -I. -Wall
In file included from /usr/local/include/c++/12.1.0/atomic:38,
                 from p721.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 \
      |  ^~~~~
p721.cpp:30:1: warning: identifier 'constexpr' is a keyword in C++11 [-Wc++11-compat]
   30 | constexpr expected();
      | ^~~~~~~~~
p721.cpp:44:36: warning: identifier 'noexcept' is a keyword in C++11 [-Wc++11-compat]
   44 | constexpr expected(expected&& rhs) noexcept(see_below);
      |                                    ^~~~~~~~
p721.cpp:30:1: error: 'constexpr' does not name a type
   30 | constexpr expected();
      | ^~~~~~~~~
p721.cpp:30:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p721.cpp:35:1: error: 'constexpr' does not name a type
   35 | constexpr expected(const expected& rhs);
      | ^~~~~~~~~
p721.cpp:35:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p721.cpp:44:1: error: 'constexpr' does not name a type
   44 | constexpr expected(expected&& rhs) noexcept(see_below);
      | ^~~~~~~~~
p721.cpp:44:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p721.cpp:55:1: error: 'constexpr' does not name a type
   55 | constexpr explicit(see_below) expected(const expected<U, G>& rhs);
      | ^~~~~~~~~
p721.cpp:55:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p721.cpp:57:1: error: 'constexpr' does not name a type
   57 | constexpr explicit(see_below) expected(expected<U, G>&& rhs);
      | ^~~~~~~~~
p721.cpp:57:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p721.cpp:78:20: error: 'T' does not name a type
   78 | template<class U = T>
      |                    ^
p721.cpp:79:3: error: 'constexpr' does not name a type
   79 |   constexpr explicit(!is_convertible_v<U, T>) expected(U&& v);
      |   ^~~~~~~~~
p721.cpp:79:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p721.cpp:88:3: error: 'constexpr' does not name a type
   88 |   constexpr explicit(!is_convertible_v<const G&, E>) expected(const unexpected<G>& e);
      |   ^~~~~~~~~
p721.cpp:88:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p721.cpp:90:3: error: 'constexpr' does not name a type
   90 |   constexpr explicit(!is_convertible_v<G, E>) expected(unexpected<G>&& e);
      |   ^~~~~~~~~
p721.cpp:90:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p721.cpp:96:15: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   96 | template<class... Args>
      |               ^~~
p721.cpp:97:1: error: 'constexpr' does not name a type
   97 | constexpr explicit expected(in_place_t, Args&&... args);
      | ^~~~~~~~~
p721.cpp:97:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p721.cpp:102:24: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
  102 | template<class U, class... Args>
      |                        ^~~
p721.cpp:103:3: error: 'constexpr' does not name a type
  103 |   constexpr explicit expected(in_place_t, initializer_list<U> il, Args&&... args);
      |   ^~~~~~~~~
p721.cpp:103:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p721.cpp:108:33: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
  108 | template<class T, class U, class... Args>
      |                                 ^~~
p721.cpp:109:1: error: 'constexpr' does not name a type
  109 | constexpr void reinit-expected (T& newval, U& oldval, Args&&... args) {
      | ^~~~~~~~~
p721.cpp:109:1: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p721.cpp:126:4: error: 'constexpr' does not name a type
  126 |    constexpr expected& operator=(const expected& rhs);
      |    ^~~~~~~~~
p721.cpp:126:4: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p721.cpp:127:15: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
  127 | template<class... Args>
      |               ^~~
p721.cpp:128:3: error: 'constexpr' does not name a type
  128 |   constexpr explicit expected(unexpect_t, Args&&... args);
      |   ^~~~~~~~~
p721.cpp:128:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p721.cpp:133:24: warning: variadic templates only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
  133 | template<class U, class... Args>
      |                        ^~~
p721.cpp:134:3: error: 'constexpr' does not name a type
  134 |   constexpr explicit expected(unexpect_t, initializer_list<U> il, Args&&... args);
      |   ^~~~~~~~~
p721.cpp:134:3: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'

$ g++ p721.cpp -std=2b -o p721g -I. -Wall
p721.cpp:30:11: error: ISO C++ forbids declaration of 'expected' with no type [-fpermissive]
   30 | constexpr expected();
      |           ^~~~~~~~
p721.cpp:35:26: error: 'expected' does not name a type
   35 | constexpr expected(const expected& rhs);
      |                          ^~~~~~~~
p721.cpp:35:11: error: ISO C++ forbids declaration of 'expected' with no type [-fpermissive]
   35 | constexpr expected(const expected& rhs);
      |           ^~~~~~~~
p721.cpp:44:11: error: ISO C++ forbids declaration of 'expected' with no type [-fpermissive]
   44 | constexpr expected(expected&& rhs) noexcept(see_below);
      |           ^~~~~~~~
p721.cpp:44:20: error: 'constexpr const int expected' redeclared as different kind of entity
   44 | constexpr expected(expected&& rhs) noexcept(see_below);
      |                    ^~~~~~~~
p721.cpp:35:11: note: previous declaration 'constexpr int expected(const int&)'
   35 | constexpr expected(const expected& rhs);
      |           ^~~~~~~~
p721.cpp:44:31: error: 'rhs' was not declared in this scope
   44 | constexpr expected(expected&& rhs) noexcept(see_below);
      |                               ^~~
p721.cpp:55:20: error: 'see_below' was not declared in this scope
   55 | constexpr explicit(see_below) expected(const expected<U, G>& rhs);
      |                    ^~~~~~~~~
p721.cpp:55:46: error: 'expected<U, G>' does not name a type
   55 | constexpr explicit(see_below) expected(const expected<U, G>& rhs);
      |                                              ^~~~~~~~~~~~~~
p721.cpp:55:31: error: ISO C++ forbids declaration of 'expected' with no type [-fpermissive]
   55 | constexpr explicit(see_below) expected(const expected<U, G>& rhs);
      |                               ^~~~~~~~
p721.cpp:57:20: error: 'see_below' was not declared in this scope
   57 | constexpr explicit(see_below) expected(expected<U, G>&& rhs);
      |                    ^~~~~~~~~
p721.cpp:57:31: error: ISO C++ forbids declaration of 'expected' with no type [-fpermissive]
   57 | constexpr explicit(see_below) expected(expected<U, G>&& rhs);
      |                               ^~~~~~~~
p721.cpp:57:40: error: 'template<class U, class G> constexpr const int expected' redeclared as different kind of entity
   57 | constexpr explicit(see_below) expected(expected<U, G>&& rhs);
      |                                        ^~~~~~~~~~~~~~
p721.cpp:35:11: note: previous declaration 'constexpr int expected(const int&)'
   35 | constexpr expected(const expected& rhs);
      |           ^~~~~~~~
p721.cpp:57:57: error: 'rhs' was not declared in this scope
   57 | constexpr explicit(see_below) expected(expected<U, G>&& rhs);
      |                                                         ^~~
p721.cpp:78:20: error: 'T' does not name a type
   78 | template<class U = T>
      |                    ^
p721.cpp:79:43: error: 'T' was not declared in this scope
   79 |   constexpr explicit(!is_convertible_v<U, T>) expected(U&& v);
      |                                           ^
p721.cpp:79:47: error: ISO C++ forbids declaration of 'expected' with no type [-fpermissive]
   79 |   constexpr explicit(!is_convertible_v<U, T>) expected(U&& v);
      |                                               ^~~~~~~~
p721.cpp:88:50: error: 'E' was not declared in this scope
   88 |   constexpr explicit(!is_convertible_v<const G&, E>) expected(const unexpected<G>& e);
      |                                                  ^
p721.cpp:88:69: error: 'unexpected' does not name a type
   88 |   constexpr explicit(!is_convertible_v<const G&, E>) expected(const unexpected<G>& e);
      |                                                                     ^~~~~~~~~~
p721.cpp:88:79: error: expected ',' or '...' before '<' token
   88 |   constexpr explicit(!is_convertible_v<const G&, E>) expected(const unexpected<G>& e);
      |                                                                               ^
p721.cpp:88:54: error: ISO C++ forbids declaration of 'expected' with no type [-fpermissive]
   88 |   constexpr explicit(!is_convertible_v<const G&, E>) expected(const unexpected<G>& e);
      |                                                      ^~~~~~~~
p721.cpp:90:43: error: 'E' was not declared in this scope
   90 |   constexpr explicit(!is_convertible_v<G, E>) expected(unexpected<G>&& e);
      |                                           ^
p721.cpp:90:47: error: ISO C++ forbids declaration of 'expected' with no type [-fpermissive]
   90 |   constexpr explicit(!is_convertible_v<G, E>) expected(unexpected<G>&& e);
      |                                               ^~~~~~~~
p721.cpp:90:56: error: 'template<class G> constexpr const int expected' redeclared as different kind of entity
   90 |   constexpr explicit(!is_convertible_v<G, E>) expected(unexpected<G>&& e);
      |                                                        ^~~~~~~~~~
p721.cpp:35:11: note: previous declaration 'constexpr int expected(const int&)'
   35 | constexpr expected(const expected& rhs);
      |           ^~~~~~~~
p721.cpp:90:56: error: 'unexpected' was not declared in this scope; did you mean 'expected'?
   90 |   constexpr explicit(!is_convertible_v<G, E>) expected(unexpected<G>&& e);
      |                                                        ^~~~~~~~~~
      |                                                        expected
p721.cpp:90:68: error: expected primary-expression before '>' token
   90 |   constexpr explicit(!is_convertible_v<G, E>) expected(unexpected<G>&& e);
      |                                                                    ^
p721.cpp:90:72: error: label 'e' referenced outside of any function
   90 |   constexpr explicit(!is_convertible_v<G, E>) expected(unexpected<G>&& e);
      |                                                                        ^
p721.cpp:97:20: error: ISO C++ forbids declaration of 'expected' with no type [-fpermissive]
   97 | constexpr explicit expected(in_place_t, Args&&... args);
      |                    ^~~~~~~~
p721.cpp:97:11: error: 'explicit' outside class declaration
   97 | constexpr explicit expected(in_place_t, Args&&... args);
      |           ^~~~~~~~
p721.cpp:103:22: error: ISO C++ forbids declaration of 'expected' with no type [-fpermissive]
  103 |   constexpr explicit expected(in_place_t, initializer_list<U> il, Args&&... args);
      |                      ^~~~~~~~
p721.cpp:103:13: error: 'explicit' outside class declaration
  103 |   constexpr explicit expected(in_place_t, initializer_list<U> il, Args&&... args);
      |             ^~~~~~~~
p721.cpp:109:22: error: expected initializer before '-' token
  109 | constexpr void reinit-expected (T& newval, U& oldval, Args&&... args) {
      |                      ^
p721.cpp:126:14: error: 'expected' does not name a type
  126 |    constexpr expected& operator=(const expected& rhs);
      |              ^~~~~~~~
p721.cpp:128:22: error: ISO C++ forbids declaration of 'expected' with no type [-fpermissive]
  128 |   constexpr explicit expected(unexpect_t, Args&&... args);
      |                      ^~~~~~~~
p721.cpp:128:13: error: 'explicit' outside class declaration
  128 |   constexpr explicit expected(unexpect_t, Args&&... args);
      |             ^~~~~~~~
p721.cpp:128:31: error: 'template<class ... Args> constexpr const int expected' redeclared as different kind of entity
  128 |   constexpr explicit expected(unexpect_t, Args&&... args);
      |                               ^~~~~~~~~~
p721.cpp:35:11: note: previous declaration 'constexpr int expected(const int&)'
   35 | constexpr expected(const expected& rhs);
      |           ^~~~~~~~
p721.cpp:128:31: error: 'unexpect_t' was not declared in this scope; did you mean 'expected'?
  128 |   constexpr explicit expected(unexpect_t, Args&&... args);
      |                               ^~~~~~~~~~
      |                               expected
p721.cpp:128:47: error: expected primary-expression before '&&' token
  128 |   constexpr explicit expected(unexpect_t, Args&&... args);
      |                                               ^~
p721.cpp:134:22: error: ISO C++ forbids declaration of 'expected' with no type [-fpermissive]
  134 |   constexpr explicit expected(unexpect_t, initializer_list<U> il, Args&&... args);
      |                      ^~~~~~~~
p721.cpp:134:13: error: 'explicit' outside class declaration
  134 |   constexpr explicit expected(unexpect_t, initializer_list<U> il, Args&&... args);
      |             ^~~~~~~~
p721.cpp:134:31: error: 'template<class U, class ... Args> constexpr const int expected' redeclared as different kind of entity
  134 |   constexpr explicit expected(unexpect_t, initializer_list<U> il, Args&&... args);
      |                               ^~~~~~~~~~
p721.cpp:35:11: note: previous declaration 'constexpr int expected(const int&)'
   35 | constexpr expected(const expected& rhs);
      |           ^~~~~~~~
p721.cpp:134:31: error: 'unexpect_t' was not declared in this scope; did you mean 'expected'?
  134 |   constexpr explicit expected(unexpect_t, initializer_list<U> il, Args&&... args);
      |                               ^~~~~~~~~~
      |                               expected
p721.cpp:134:63: error: expected primary-expression before 'il'
  134 |   constexpr explicit expected(unexpect_t, initializer_list<U> il, Args&&... args);
      |                                                               ^~
p721.cpp:134:71: error: expected primary-expression before '&&' token
  134 |   constexpr explicit expected(unexpect_t, initializer_list<U> il, Args&&... args);
      |                                                                       ^~
```

## 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 　20220728