11.4.8.3 Conversion functions [class.conv.fct]  C++N4910:2022 (149) p279.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.

# 11.4.8.3 Conversion functions [class.conv.fct]  C++N4910:2022 (149) p279.cpp

## 算譜(source code)

```c++:p279.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 = "11.4.8.3 Conversion functions [class.conv.fct]  C++N4910:2022 (149) p279.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>

using namespace std;

// Example 1
struct X {
    operator int();
    operator auto() -> short;
};
void f(X a) {
    int i = int(a);
    i = (int)a;
    i = a;
// error: trailing return type
}
// In all three cases the value assigned will be converted by X::operator int().
// [Example 2:
class Y { };
struct Z {
    explicit operator Y() const;
};
void h(Z z) {
    Y y1(z);
    Y y2 = z;
    Y y3 = (Y)z;
}
// OK, direct-initialization
// error: no conversion function candidate for copy-initialization // OK, cast notation
void g(X a, X b) {
    int i = (a) ? 1+a : 0;
    int j = (a&&b) ? a+b : i;
    if (a) {
    }
}
// [Example 3:
&ac.operator int*i; // syntax error:
// parsed as: &(ac.operator int *)i
// not as: &(ac.operator int)*i
// The * is the pointer declarator and not the multiplication operator.
// [Example 4:
operator int [[noreturn]] (); // error: noreturn attribute applied to a type
// [Example 5:
struct X5 {
    operator int();
};
struct Y5 : X5 {
    operator char();
};
void f5(Y5& a) {
    if (a) {
    }
}
// error: ambiguous between X::operator int() and Y::operator char()
// [Example 6:
struct S {
    operator auto() const {
        return 10;
    }
    template<class T>
    operator auto() const {
        return 1.2;
    }
};
// OK
// error: conversion function template
int main() {
    cout  <<  n4910 << endl;
    return EXIT_SUCCESS;
}
```

## Script

```bash:clgc.sh
#!/bin/sh
rm $1l
rm $1g
echo "$ clang++ $1.cpp -std=03 -o $1l -I. -Wall" 
clang++ $1.cpp -std=c++03 -o $1l -I. -Wall
if [  -e $1l ]; then
./$1l 
fi
rm $1l
echo "$ clang++ $1.cpp -std=2b -o $1l -I. -Wall"
clang++ $1.cpp -std=c++2b -o $1l -I. -Wall
if [  -e $1l ]; then
./$1l
fi
echo "\r"
echo "$ g++ $1.cpp -std=03 -o $1g -I. -Wall"
g++ $1.cpp -std=c++03 -o $1g -I. -Wall
if [ -e $1g ]; then
./$1g
fi
rm $1g
echo "\r"
echo "$ g++ $1.cpp -std=2b -o $1g -I. -Wall"
g++ $1.cpp -std=c++2b -o $1g -I. -Wall
if [ -e $1g ]; then
./$1g 
fi
```
## 編纂・実行結果(compile and go)

```ubuntu:bash
# ./clgc.sh  p279
rm: cannot remove 'p279l': No such file or directory
rm: cannot remove 'p279g': No such file or directory
$ clang++ p279.cpp -std=c++03 -o p279l -I. -Wall
In file included from p279.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 \
 ^
p279.cpp:27:14: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
    operator auto() -> short;
             ^
p279.cpp:27:14: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
p279.cpp:27:14: error: 'auto' not allowed in conversion function type
    operator auto() -> short;
             ^~~~
p279.cpp:30:9: warning: variable 'i' set but not used [-Wunused-but-set-variable]
    int i = int(a);
        ^
p279.cpp:39:8: warning: explicit conversion functions are a C++11 extension [-Wc++11-extensions]
       explicit operator Y() const;
       ^~~~~~~~
p279.cpp:42:5: error: no matching constructor for initialization of 'Y'
  Y y1(z);
    ^  ~
p279.cpp:37:12: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'Z' to 'const Y' for 1st argument
     class Y { };
           ^
p279.cpp:37:12: note: candidate constructor (the implicit default constructor) not viable: requires 0 arguments, but 1 was provided
p279.cpp:43:5: error: no viable conversion from 'Z' to 'Y'
  Y y2 = z;
    ^    ~
p279.cpp:37:12: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'Z' to 'const Y &' for 1st argument
     class Y { };
           ^
p279.cpp:39:17: note: explicit conversion function is not a candidate
       explicit operator Y() const;
                ^
p279.cpp:44:10: error: no matching conversion for C-style cast from 'Z' to 'Y'
  Y y3 = (Y)z;
         ^~~~
p279.cpp:37:12: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'Z' to 'const Y' for 1st argument
     class Y { };
           ^
p279.cpp:37:12: note: candidate constructor (the implicit default constructor) not viable: requires 0 arguments, but 1 was provided
p279.cpp:50:12: warning: unused variable 'j' [-Wunused-variable]
       int j = (a&&b) ? a+b : i;
           ^
p279.cpp:55:2: error: C++ requires a type specifier for all declarations
&ac.operator int*i; // syntax error:
 ^
p279.cpp:55:2: error: declaration of reference variable 'ac' requires an initializer
&ac.operator int*i; // syntax error:
 ^~
p279.cpp:55:4: error: expected ';' after top level declarator
&ac.operator int*i; // syntax error:
   ^
   ;
p279.cpp:60:15: error: expected expression
operator int [[noreturn]] (); // error: noreturn attribute applied to a type 
              ^
p279.cpp:60:1: error: conversion function must be a non-static member function
operator int [[noreturn]] (); // error: noreturn attribute applied to a type 
^
p279.cpp:69:9: error: conversion from 'Y5' to 'bool' is ambiguous
    if (a) {
        ^
p279.cpp:66:5: note: candidate function
    operator char();
    ^
p279.cpp:63:3: note: candidate function
  operator int();
  ^
p279.cpp:75:14: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
    operator auto() const { return 10; }
             ^
p279.cpp:75:14: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
p279.cpp:75:14: error: 'auto' not allowed in conversion function type
    operator auto() const { return 10; }
             ^~~~
7 warnings and 12 errors generated.
rm: cannot remove 'p279l': No such file or directory
$ clang++ p279.cpp -std=c++2b -o p279l -I. -Wall
p279.cpp:27:24: error: cannot specify any part of a return type in the declaration of a conversion function; put the complete type after 'operator'
    operator auto() -> short;
                       ^~~~~
p279.cpp:30:9: warning: variable 'i' set but not used [-Wunused-but-set-variable]
    int i = int(a);
        ^
p279.cpp:43:5: error: no viable conversion from 'Z' to 'Y'
  Y y2 = z;
    ^    ~
p279.cpp:37:12: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'Z' to 'const Y &' for 1st argument
     class Y { };
           ^
p279.cpp:37:12: note: candidate constructor (the implicit move constructor) not viable: no known conversion from 'Z' to 'Y &&' for 1st argument
     class Y { };
           ^
p279.cpp:39:17: note: explicit conversion function is not a candidate
       explicit operator Y() const;
                ^
p279.cpp:49:23: error: use of overloaded operator '+' is ambiguous (with operand types 'int' and 'X')
       int i = (a) ? 1+a : 0;
                     ~^~
p279.cpp:49:23: note: because of ambiguity in conversion of 'X' to 'float'
p279.cpp:26:5: note: candidate function
    operator int();
    ^
p279.cpp:27:5: note: candidate function
    operator auto() -> short;
    ^
p279.cpp:49:23: note: built-in candidate operator+(int, float)
       int i = (a) ? 1+a : 0;
                      ^
p279.cpp:49:23: note: built-in candidate operator+(int, double)
p279.cpp:49:23: note: built-in candidate operator+(int, long double)
p279.cpp:49:23: note: built-in candidate operator+(int, __float128)
p279.cpp:49:23: note: built-in candidate operator+(int, int)
p279.cpp:49:23: note: built-in candidate operator+(int, long)
p279.cpp:49:23: note: built-in candidate operator+(int, long long)
p279.cpp:49:23: note: built-in candidate operator+(int, __int128)
p279.cpp:49:23: note: built-in candidate operator+(int, unsigned int)
p279.cpp:49:23: note: built-in candidate operator+(int, unsigned long)
p279.cpp:49:23: note: built-in candidate operator+(int, unsigned long long)
p279.cpp:49:23: note: built-in candidate operator+(int, unsigned __int128)
p279.cpp:49:16: error: conversion from 'X' to 'bool' is ambiguous
       int i = (a) ? 1+a : 0;
               ^~~
p279.cpp:26:5: note: candidate function
    operator int();
    ^
p279.cpp:27:5: note: candidate function
    operator auto() -> short;
    ^
p279.cpp:50:17: error: conversion from 'X' to 'bool' is ambiguous
       int j = (a&&b) ? a+b : i;
                ^
p279.cpp:26:5: note: candidate function
    operator int();
    ^
p279.cpp:27:5: note: candidate function
    operator auto() -> short;
    ^
p279.cpp:50:26: error: use of overloaded operator '+' is ambiguous (with operand types 'X' and 'X')
       int j = (a&&b) ? a+b : i;
                        ~^~
p279.cpp:50:26: note: because of ambiguity in conversion of 'X' to 'float'
p279.cpp:26:5: note: candidate function
    operator int();
    ^
p279.cpp:27:5: note: candidate function
    operator auto() -> short;
    ^
p279.cpp:50:26: note: because of ambiguity in conversion of 'X' to 'float'
       int j = (a&&b) ? a+b : i;
                         ^
p279.cpp:26:5: note: candidate function
    operator int();
    ^
p279.cpp:27:5: note: candidate function
    operator auto() -> short;
    ^
p279.cpp:50:26: note: built-in candidate operator+(float, float)
       int j = (a&&b) ? a+b : i;
                         ^
p279.cpp:50:26: note: built-in candidate operator+(float, double)
p279.cpp:50:26: note: built-in candidate operator+(float, long double)
p279.cpp:50:26: note: built-in candidate operator+(float, __float128)
p279.cpp:50:26: note: built-in candidate operator+(float, int)
p279.cpp:50:26: note: built-in candidate operator+(float, long)
p279.cpp:50:26: note: built-in candidate operator+(float, long long)
p279.cpp:50:26: note: built-in candidate operator+(float, __int128)
p279.cpp:50:26: note: built-in candidate operator+(float, unsigned int)
p279.cpp:50:26: note: built-in candidate operator+(float, unsigned long)
p279.cpp:50:26: note: built-in candidate operator+(float, unsigned long long)
p279.cpp:50:26: note: built-in candidate operator+(float, unsigned __int128)
p279.cpp:50:26: note: built-in candidate operator+(double, float)
p279.cpp:50:26: note: built-in candidate operator+(double, double)
p279.cpp:50:26: note: built-in candidate operator+(double, long double)
p279.cpp:50:26: note: built-in candidate operator+(double, __float128)
p279.cpp:50:26: note: built-in candidate operator+(double, int)
p279.cpp:50:26: note: built-in candidate operator+(double, long)
p279.cpp:50:26: note: built-in candidate operator+(double, long long)
p279.cpp:50:26: note: built-in candidate operator+(double, __int128)
p279.cpp:50:26: note: built-in candidate operator+(double, unsigned int)
p279.cpp:50:26: note: built-in candidate operator+(double, unsigned long)
p279.cpp:50:26: note: built-in candidate operator+(double, unsigned long long)
p279.cpp:50:26: note: built-in candidate operator+(double, unsigned __int128)
p279.cpp:50:26: note: built-in candidate operator+(long double, float)
p279.cpp:50:26: note: built-in candidate operator+(long double, double)
p279.cpp:50:26: note: built-in candidate operator+(long double, long double)
p279.cpp:50:26: note: built-in candidate operator+(long double, __float128)
p279.cpp:50:26: note: built-in candidate operator+(long double, int)
p279.cpp:50:26: note: built-in candidate operator+(long double, long)
p279.cpp:50:26: note: built-in candidate operator+(long double, long long)
p279.cpp:50:26: note: built-in candidate operator+(long double, __int128)
p279.cpp:50:26: note: built-in candidate operator+(long double, unsigned int)
p279.cpp:50:26: note: built-in candidate operator+(long double, unsigned long)
p279.cpp:50:26: note: built-in candidate operator+(long double, unsigned long long)
p279.cpp:50:26: note: built-in candidate operator+(long double, unsigned __int128)
p279.cpp:50:26: note: built-in candidate operator+(__float128, float)
p279.cpp:50:26: note: built-in candidate operator+(__float128, double)
p279.cpp:50:26: note: built-in candidate operator+(__float128, long double)
p279.cpp:50:26: note: built-in candidate operator+(__float128, __float128)
p279.cpp:50:26: note: built-in candidate operator+(__float128, int)
p279.cpp:50:26: note: built-in candidate operator+(__float128, long)
p279.cpp:50:26: note: built-in candidate operator+(__float128, long long)
p279.cpp:50:26: note: built-in candidate operator+(__float128, __int128)
p279.cpp:50:26: note: built-in candidate operator+(__float128, unsigned int)
p279.cpp:50:26: note: built-in candidate operator+(__float128, unsigned long)
p279.cpp:50:26: note: built-in candidate operator+(__float128, unsigned long long)
p279.cpp:50:26: note: built-in candidate operator+(__float128, unsigned __int128)
p279.cpp:50:26: note: built-in candidate operator+(int, float)
p279.cpp:50:26: note: built-in candidate operator+(int, double)
p279.cpp:50:26: note: built-in candidate operator+(int, long double)
p279.cpp:50:26: note: built-in candidate operator+(int, __float128)
p279.cpp:50:26: note: built-in candidate operator+(int, int)
p279.cpp:50:26: note: built-in candidate operator+(int, long)
p279.cpp:50:26: note: built-in candidate operator+(int, long long)
p279.cpp:50:26: note: built-in candidate operator+(int, __int128)
p279.cpp:50:26: note: built-in candidate operator+(int, unsigned int)
p279.cpp:50:26: note: built-in candidate operator+(int, unsigned long)
p279.cpp:50:26: note: built-in candidate operator+(int, unsigned long long)
p279.cpp:50:26: note: built-in candidate operator+(int, unsigned __int128)
p279.cpp:50:26: note: built-in candidate operator+(long, float)
p279.cpp:50:26: note: built-in candidate operator+(long, double)
p279.cpp:50:26: note: built-in candidate operator+(long, long double)
p279.cpp:50:26: note: built-in candidate operator+(long, __float128)
p279.cpp:50:26: note: built-in candidate operator+(long, int)
p279.cpp:50:26: note: built-in candidate operator+(long, long)
p279.cpp:50:26: note: built-in candidate operator+(long, long long)
p279.cpp:50:26: note: built-in candidate operator+(long, __int128)
p279.cpp:50:26: note: built-in candidate operator+(long, unsigned int)
p279.cpp:50:26: note: built-in candidate operator+(long, unsigned long)
p279.cpp:50:26: note: built-in candidate operator+(long, unsigned long long)
p279.cpp:50:26: note: built-in candidate operator+(long, unsigned __int128)
p279.cpp:50:26: note: built-in candidate operator+(long long, float)
p279.cpp:50:26: note: built-in candidate operator+(long long, double)
p279.cpp:50:26: note: built-in candidate operator+(long long, long double)
p279.cpp:50:26: note: built-in candidate operator+(long long, __float128)
p279.cpp:50:26: note: built-in candidate operator+(long long, int)
p279.cpp:50:26: note: built-in candidate operator+(long long, long)
p279.cpp:50:26: note: built-in candidate operator+(long long, long long)
p279.cpp:50:26: note: built-in candidate operator+(long long, __int128)
p279.cpp:50:26: note: built-in candidate operator+(long long, unsigned int)
p279.cpp:50:26: note: built-in candidate operator+(long long, unsigned long)
p279.cpp:50:26: note: built-in candidate operator+(long long, unsigned long long)
p279.cpp:50:26: note: built-in candidate operator+(long long, unsigned __int128)
p279.cpp:50:26: note: built-in candidate operator+(__int128, float)
p279.cpp:50:26: note: built-in candidate operator+(__int128, double)
p279.cpp:50:26: note: built-in candidate operator+(__int128, long double)
p279.cpp:50:26: note: built-in candidate operator+(__int128, __float128)
p279.cpp:50:26: note: built-in candidate operator+(__int128, int)
p279.cpp:50:26: note: built-in candidate operator+(__int128, long)
p279.cpp:50:26: note: built-in candidate operator+(__int128, long long)
p279.cpp:50:26: note: built-in candidate operator+(__int128, __int128)
p279.cpp:50:26: note: built-in candidate operator+(__int128, unsigned int)
p279.cpp:50:26: note: built-in candidate operator+(__int128, unsigned long)
p279.cpp:50:26: note: built-in candidate operator+(__int128, unsigned long long)
p279.cpp:50:26: note: built-in candidate operator+(__int128, unsigned __int128)
p279.cpp:50:26: note: built-in candidate operator+(unsigned int, float)
p279.cpp:50:26: note: built-in candidate operator+(unsigned int, double)
p279.cpp:50:26: note: built-in candidate operator+(unsigned int, long double)
p279.cpp:50:26: note: built-in candidate operator+(unsigned int, __float128)
p279.cpp:50:26: note: built-in candidate operator+(unsigned int, int)
p279.cpp:50:26: note: built-in candidate operator+(unsigned int, long)
p279.cpp:50:26: note: built-in candidate operator+(unsigned int, long long)
p279.cpp:50:26: note: built-in candidate operator+(unsigned int, __int128)
p279.cpp:50:26: note: built-in candidate operator+(unsigned int, unsigned int)
p279.cpp:50:26: note: built-in candidate operator+(unsigned int, unsigned long)
p279.cpp:50:26: note: built-in candidate operator+(unsigned int, unsigned long long)
p279.cpp:50:26: note: built-in candidate operator+(unsigned int, unsigned __int128)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long, float)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long, double)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long, long double)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long, __float128)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long, int)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long, long)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long, long long)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long, __int128)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long, unsigned int)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long, unsigned long)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long, unsigned long long)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long, unsigned __int128)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long long, float)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long long, double)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long long, long double)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long long, __float128)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long long, int)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long long, long)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long long, long long)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long long, __int128)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long long, unsigned int)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long long, unsigned long)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long long, unsigned long long)
p279.cpp:50:26: note: built-in candidate operator+(unsigned long long, unsigned __int128)
p279.cpp:50:26: note: built-in candidate operator+(unsigned __int128, float)
p279.cpp:50:26: note: built-in candidate operator+(unsigned __int128, double)
p279.cpp:50:26: note: built-in candidate operator+(unsigned __int128, long double)
p279.cpp:50:26: note: built-in candidate operator+(unsigned __int128, __float128)
p279.cpp:50:26: note: built-in candidate operator+(unsigned __int128, int)
p279.cpp:50:26: note: built-in candidate operator+(unsigned __int128, long)
p279.cpp:50:26: note: built-in candidate operator+(unsigned __int128, long long)
p279.cpp:50:26: note: built-in candidate operator+(unsigned __int128, __int128)
p279.cpp:50:26: note: built-in candidate operator+(unsigned __int128, unsigned int)
p279.cpp:50:26: note: built-in candidate operator+(unsigned __int128, unsigned long)
p279.cpp:50:26: note: built-in candidate operator+(unsigned __int128, unsigned long long)
p279.cpp:50:26: note: built-in candidate operator+(unsigned __int128, unsigned __int128)
p279.cpp:51:12: error: conversion from 'X' to 'bool' is ambiguous
       if (a) {
           ^
p279.cpp:26:5: note: candidate function
    operator int();
    ^
p279.cpp:27:5: note: candidate function
    operator auto() -> short;
    ^
p279.cpp:55:2: error: C++ requires a type specifier for all declarations
&ac.operator int*i; // syntax error:
 ^
p279.cpp:55:2: error: declaration of reference variable 'ac' requires an initializer
&ac.operator int*i; // syntax error:
 ^~
p279.cpp:55:4: error: expected ';' after top level declarator
&ac.operator int*i; // syntax error:
   ^
   ;
p279.cpp:60:16: error: 'noreturn' attribute cannot be applied to types
operator int [[noreturn]] (); // error: noreturn attribute applied to a type 
               ^
p279.cpp:60:1: error: conversion function must be a non-static member function
operator int [[noreturn]] (); // error: noreturn attribute applied to a type 
^
p279.cpp:69:9: error: conversion from 'Y5' to 'bool' is ambiguous
    if (a) {
        ^
p279.cpp:66:5: note: candidate function
    operator char();
    ^
p279.cpp:63:3: note: candidate function
  operator int();
  ^
1 warning and 13 errors generated.

$ g++ p279.cpp -std=c++03 -o p279g -I. -Wall
In file included from /usr/local/include/c++/12.1.0/atomic:38,
                 from p279.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 \
      |  ^~~~~
p279.cpp:27:5: warning: C++11 auto only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   27 |     operator auto() -> short;
      |     ^~~~~~~~
p279.cpp:27:5: warning: C++11 auto only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
p279.cpp:27:14: error: invalid use of 'auto' in conversion operator
   27 |     operator auto() -> short;
      |              ^~~~
p279.cpp:39:30: warning: explicit conversion operators only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   39 |        explicit operator Y() const;
      |                              ^~~~~
p279.cpp: In function 'void h(Z)':
p279.cpp:43:10: error: conversion from 'Z' to non-scalar type 'Y' requested
   43 |   Y y2 = z;
      |          ^
p279.cpp:43:5: warning: unused variable 'y2' [-Wunused-variable]
   43 |   Y y2 = z;
      |     ^~
p279.cpp: In function 'void g(X, X)':
p279.cpp:50:12: warning: unused variable 'j' [-Wunused-variable]
   50 |        int j = (a&&b) ? a+b : i;
      |            ^
p279.cpp: At global scope:
p279.cpp:55:4: error: expected constructor, destructor, or type conversion before '.' token
   55 | &ac.operator int*i; // syntax error:
      |    ^
p279.cpp:60:1: error: 'operator int' does not name a type
   60 | operator int [[noreturn]] (); // error: noreturn attribute applied to a type
      | ^~~~~~~~
p279.cpp: In function 'void f5(Y5&)':
p279.cpp:69:9: error: conversion from 'Y5' to 'bool' is ambiguous
   69 |     if (a) {
      |         ^
p279.cpp:66:5: note: candidate: 'Y5::operator char()'
   66 |     operator char();
      |     ^~~~~~~~
p279.cpp:63:3: note: candidate: 'X5::operator int()'
   63 |   operator int();
      |   ^~~~~~~~
p279.cpp: At global scope:
p279.cpp:75:5: warning: C++11 auto only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   75 |     operator auto() const { return 10; }
      |     ^~~~~~~~
p279.cpp:75:5: warning: C++11 auto only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
p279.cpp:75:14: error: invalid use of 'auto' in conversion operator
   75 |     operator auto() const { return 10; }
      |              ^~~~
p279.cpp:77:5: warning: C++11 auto only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   77 |     operator auto() const { return 1.2; }
      |     ^~~~~~~~
p279.cpp:77:5: warning: C++11 auto only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
p279.cpp:77:14: error: invalid use of 'auto' in conversion operator
   77 |     operator auto() const { return 1.2; }
      |              ^~~~
rm: cannot remove 'p279g': No such file or directory

$ g++ p279.cpp -std=c++2b -o p279g -I. -Wall
p279.cpp:27:24: error: a conversion function cannot have a trailing return type
   27 |     operator auto() -> short;
      |                        ^~~~~
p279.cpp: In function 'void h(Z)':
p279.cpp:43:10: error: conversion from 'Z' to non-scalar type 'Y' requested
   43 |   Y y2 = z;
      |          ^
p279.cpp:43:5: warning: unused variable 'y2' [-Wunused-variable]
   43 |   Y y2 = z;
      |     ^~
p279.cpp: In function 'void g(X, X)':
p279.cpp:50:12: warning: unused variable 'j' [-Wunused-variable]
   50 |        int j = (a&&b) ? a+b : i;
      |            ^
p279.cpp: At global scope:
p279.cpp:55:4: error: expected constructor, destructor, or type conversion before '.' token
   55 | &ac.operator int*i; // syntax error:
      |    ^
p279.cpp:60:29: error: expected constructor, destructor, or type conversion before ';' token
   60 | operator int [[noreturn]] (); // error: noreturn attribute applied to a type
      |                             ^
p279.cpp: In function 'void f5(Y5&)':
p279.cpp:69:9: error: conversion from 'Y5' to 'bool' is ambiguous
   69 |     if (a) {
      |         ^
p279.cpp:66:5: note: candidate: 'Y5::operator char()'
   66 |     operator char();
      |     ^~~~~~~~
p279.cpp:63:3: note: candidate: 'X5::operator int()'
   63 |   operator int();
      |   ^~~~~~~~
p279.cpp: At global scope:
p279.cpp:77:14: warning: use of 'auto' in member template conversion operator can never be deduced
   77 |     operator auto() const { return 1.2; }
      |              ^~~~
```

## 検討事項(agenda)
コンパイルエラーの理由を解説する。

<|--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

## 自己参照(self reference)

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


 ＜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 　20220629