9.2.9.6.2 Placeholder type deduction [dcl.type.auto.deduct] C++N4910:2022 (90) p183.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.

# 9.2.9.6.2 Placeholder type deduction [dcl.type.auto.deduct] C++N4910:2022 (90) p183.cpp

## 算譜(source code)

```c++:p183.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 * msg="9.2.9.6.2 Placeholder type deduction [dcl.type.auto.deduct] C++N4910:2022 (90) p183.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 <cstdlib> 
#include <cstring>
#include <cassert>
#include <coroutine>

using namespace std;

// Example 1  
auto x1 = { 1, 2 }; // decltype(x1) is std::initializer_list<int>
auto x2 = { 1, 2.0 }; // error: cannot deduce element type
auto x3{ 1, 2 }; // error: not a single element
auto x4 = { 3 }; // decltype(x4) is std::initializer_list<int>
auto x5{ 3 }; // decltype(x5) is int
// [Example 2 :
const auto &i = expr;
The type of i is the deduced type of the parameter u in the call f(expr) of the following invented function template:
template <class U> void f(const U& u);
// [Example 3 :
int i;
int&& f();
auto x2a(i); // decltype(x2a) is int
decltype(auto) x2d(i); // decltype(x2d) is int
auto x3a = i; // decltype(x3a) is int
decltype(auto) x3d = i; // decltype(x3d) is int
auto x4a = (i); // decltype(x4a) is int
decltype(auto) x4d = (i); // decltype(x4d) is int&
auto x5a = f(); // decltype(x5a) is int
decltype(auto) x5d = f(); // decltype(x5d) is int&&
auto x6a = { 1, 2 }; // decltype(x6a) is std::initializer_list<int>
decltype(auto) x6d = { 1, 2 }; // error: { 1, 2 } is not an expression
auto *x7a = &i; // decltype(x7a) is int*
decltype(auto)*x7d = &i; // error: declared type is not plain decltype(auto)
int main(){
  cout  <<  msg << 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  p183
rm: cannot remove 'p183l': No such file or directory
rm: cannot remove 'p183g': No such file or directory
$ clang++ p183.cpp -std=03 -o p183l -I. -Wall
p183.cpp:19:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto x1 = { 1, 2 }; // decltype(x1) is std::initializer_list<int>
^
p183.cpp:19:11: error: cannot deduce type of initializer list because std::initializer_list was not found; include <initializer_list>
auto x1 = { 1, 2 }; // decltype(x1) is std::initializer_list<int>
          ^
p183.cpp:20:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto x2 = { 1, 2.0 }; // error: cannot deduce element type
^
p183.cpp:20:11: error: deduced conflicting types ('int' vs 'double') for initializer list element type
auto x2 = { 1, 2.0 }; // error: cannot deduce element type
          ^ ~  ~~~
p183.cpp:21:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto x3{ 1, 2 }; // error: not a single element
^
p183.cpp:21:6: error: declaration of variable 'x3' with deduced type 'auto' requires an initializer
auto x3{ 1, 2 }; // error: not a single element
     ^
p183.cpp:21:8: error: expected ';' after top level declarator
auto x3{ 1, 2 }; // error: not a single element
       ^
       ;
p183.cpp:22:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto x4 = { 3 }; // decltype(x4) is std::initializer_list<int>
^
p183.cpp:22:11: error: cannot deduce type of initializer list because std::initializer_list was not found; include <initializer_list>
auto x4 = { 3 }; // decltype(x4) is std::initializer_list<int>
          ^
p183.cpp:23:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto x5{ 3 }; // decltype(x5) is int
^
p183.cpp:23:6: error: declaration of variable 'x5' with deduced type 'auto' requires an initializer
auto x5{ 3 }; // decltype(x5) is int
     ^
p183.cpp:23:8: error: expected ';' after top level declarator
auto x5{ 3 }; // decltype(x5) is int
       ^
       ;
p183.cpp:25:7: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
const auto &i = expr;
      ^
p183.cpp:25:17: error: use of undeclared identifier 'expr'; did you mean 'exit'?
const auto &i = expr;
                ^~~~
                exit
/usr/include/stdlib.h:617:13: note: 'exit' declared here
extern void exit (int __status) __THROW __attribute__ ((__noreturn__));
            ^
p183.cpp:29:5: error: redefinition of 'i' with a different type: 'int' vs 'void (&)(int) __attribute__((noreturn)) throw()'
int i;
    ^
p183.cpp:25:13: note: previous definition is here
const auto &i = expr;
            ^
p183.cpp:30:4: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
int&& f();
   ^
p183.cpp:31:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto x2a(i); // decltype(x2a) is int
^
p183.cpp:32:10: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
decltype(auto) x2d(i); // decltype(x2d) is int
         ^
p183.cpp:32:10: error: 'auto' not allowed in function prototype
decltype(auto) x2d(i); // decltype(x2d) is int
         ^~~~
p183.cpp:32:16: error: expected function body after function declarator
decltype(auto) x2d(i); // decltype(x2d) is int
               ^
p183.cpp:33:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto x3a = i; // decltype(x3a) is int
^
p183.cpp:34:10: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
decltype(auto) x3d = i; // decltype(x3d) is int
         ^
p183.cpp:34:10: error: 'auto' not allowed in function prototype
decltype(auto) x3d = i; // decltype(x3d) is int
         ^~~~
p183.cpp:34:16: error: expected function body after function declarator
decltype(auto) x3d = i; // decltype(x3d) is int
               ^
p183.cpp:35:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto x4a = (i); // decltype(x4a) is int
^
p183.cpp:36:10: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
decltype(auto) x4d = (i); // decltype(x4d) is int&
         ^
p183.cpp:36:10: error: 'auto' not allowed in function prototype
decltype(auto) x4d = (i); // decltype(x4d) is int&
         ^~~~
p183.cpp:36:16: error: expected function body after function declarator
decltype(auto) x4d = (i); // decltype(x4d) is int&
               ^
p183.cpp:37:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto x5a = f(); // decltype(x5a) is int
^
p183.cpp:38:10: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
decltype(auto) x5d = f(); // decltype(x5d) is int&&
         ^
p183.cpp:38:10: error: 'auto' not allowed in function prototype
decltype(auto) x5d = f(); // decltype(x5d) is int&&
         ^~~~
p183.cpp:38:16: error: expected function body after function declarator
decltype(auto) x5d = f(); // decltype(x5d) is int&&
               ^
p183.cpp:39:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto x6a = { 1, 2 }; // decltype(x6a) is std::initializer_list<int>
^
p183.cpp:39:12: error: cannot deduce type of initializer list because std::initializer_list was not found; include <initializer_list>
auto x6a = { 1, 2 }; // decltype(x6a) is std::initializer_list<int>
           ^
p183.cpp:40:10: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
decltype(auto) x6d = { 1, 2 }; // error: { 1, 2 } is not an expression
         ^
p183.cpp:40:10: error: 'auto' not allowed in function prototype
decltype(auto) x6d = { 1, 2 }; // error: { 1, 2 } is not an expression
         ^~~~
fatal error: too many errors emitted, stopping now [-ferror-limit=]
17 warnings and 20 errors generated.
rm: cannot remove 'p183l': No such file or directory
$ clang++ p183.cpp -std=2b -o p183l -I. -Wall
p183.cpp:20:11: error: deduced conflicting types ('int' vs 'double') for initializer list element type
auto x2 = { 1, 2.0 }; // error: cannot deduce element type
          ^ ~  ~~~
p183.cpp:21:13: error: initializer for variable 'x3' with type 'auto' contains multiple expressions
auto x3{ 1, 2 }; // error: not a single element
~~~~~~~     ^
p183.cpp:25:17: error: use of undeclared identifier 'expr'; did you mean 'exit'?
const auto &i = expr;
                ^~~~
                exit
/usr/include/stdlib.h:617:13: note: 'exit' declared here
extern void exit (int __status) __THROW __attribute__ ((__noreturn__));
            ^
p183.cpp:29:5: error: redefinition of 'i' with a different type: 'int' vs 'void (&)(int) __attribute__((noreturn)) throw()'
int i;
    ^
p183.cpp:25:13: note: previous definition is here
const auto &i = expr;
            ^
p183.cpp:40:22: error: cannot deduce 'decltype(auto)' from initializer list
decltype(auto) x6d = { 1, 2 }; // error: { 1, 2 } is not an expression
                     ^
p183.cpp:42:15: error: cannot form pointer to 'decltype(auto)'
decltype(auto)*x7d = &i; // error: declared type is not plain decltype(auto)
              ^
6 errors generated.

$ g++ p183.cpp -std=03 -o p183g -I. -Wall
p183.cpp:32:1: warning: identifier 'decltype' is a keyword in C++11 [-Wc++11-compat]
   32 | decltype(auto) x2d(i); // decltype(x2d) is int
      | ^~~~~~~~
p183.cpp:19:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   19 | auto x1 = { 1, 2 }; // decltype(x1) is std::initializer_list<int>
      | ^~~~
      | ----
p183.cpp:19:6: error: 'x1' does not name a type
   19 | auto x1 = { 1, 2 }; // decltype(x1) is std::initializer_list<int>
      |      ^~
p183.cpp:20:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   20 | auto x2 = { 1, 2.0 }; // error: cannot deduce element type
      | ^~~~
      | ----
p183.cpp:20:6: error: 'x2' does not name a type
   20 | auto x2 = { 1, 2.0 }; // error: cannot deduce element type
      |      ^~
p183.cpp:21:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   21 | auto x3{ 1, 2 }; // error: not a single element
      | ^~~~
      | ----
p183.cpp:21:6: error: 'x3' does not name a type
   21 | auto x3{ 1, 2 }; // error: not a single element
      |      ^~
p183.cpp:22:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   22 | auto x4 = { 3 }; // decltype(x4) is std::initializer_list<int>
      | ^~~~
      | ----
p183.cpp:22:6: error: 'x4' does not name a type
   22 | auto x4 = { 3 }; // decltype(x4) is std::initializer_list<int>
      |      ^~
p183.cpp:23:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   23 | auto x5{ 3 }; // decltype(x5) is int
      | ^~~~
      | ----
p183.cpp:23:6: error: 'x5' does not name a type
   23 | auto x5{ 3 }; // decltype(x5) is int
      |      ^~
p183.cpp:25:7: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   25 | const auto &i = expr;
      |       ^~~~
      |       ----
p183.cpp:25:13: error: ISO C++ forbids declaration of 'i' with no type [-fpermissive]
   25 | const auto &i = expr;
      |             ^
p183.cpp:25:7: error: top-level declaration of 'i' specifies 'auto'
   25 | const auto &i = expr;
      |       ^~~~
p183.cpp:25:17: error: 'expr' was not declared in this scope
   25 | const auto &i = expr;
      |                 ^~~~
p183.cpp:29:5: error: conflicting declaration 'int i'
   29 | int i;
      |     ^
p183.cpp:25:13: note: previous declaration as 'const int& i'
   25 | const auto &i = expr;
      |             ^
p183.cpp:30:4: error: expected unqualified-id before '&&' token
   30 | int&& f();
      |    ^~
p183.cpp:31:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   31 | auto x2a(i); // decltype(x2a) is int
      | ^~~~
      | ----
p183.cpp:31:6: error: ISO C++ forbids declaration of 'x2a' with no type [-fpermissive]
   31 | auto x2a(i); // decltype(x2a) is int
      |      ^~~
p183.cpp:31:1: error: top-level declaration of 'x2a' specifies 'auto'
   31 | auto x2a(i); // decltype(x2a) is int
      | ^~~~
p183.cpp:32:10: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   32 | decltype(auto) x2d(i); // decltype(x2d) is int
      |          ^~~~
      |          ----
p183.cpp:32:10: error: ISO C++ forbids declaration of 'parameter' with no type [-fpermissive]
p183.cpp:32:16: error: expected constructor, destructor, or type conversion before 'x2d'
   32 | decltype(auto) x2d(i); // decltype(x2d) is int
      |                ^~~
p183.cpp:33:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   33 | auto x3a = i; // decltype(x3a) is int
      | ^~~~
      | ----
p183.cpp:33:6: error: 'x3a' does not name a type
   33 | auto x3a = i; // decltype(x3a) is int
      |      ^~~
p183.cpp:34:10: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   34 | decltype(auto) x3d = i; // decltype(x3d) is int
      |          ^~~~
      |          ----
p183.cpp:34:10: error: ISO C++ forbids declaration of 'parameter' with no type [-fpermissive]
p183.cpp:34:16: error: expected constructor, destructor, or type conversion before 'x3d'
   34 | decltype(auto) x3d = i; // decltype(x3d) is int
      |                ^~~
p183.cpp:35:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   35 | auto x4a = (i); // decltype(x4a) is int
      | ^~~~
      | ----
p183.cpp:35:6: error: 'x4a' does not name a type
   35 | auto x4a = (i); // decltype(x4a) is int
      |      ^~~
p183.cpp:36:10: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   36 | decltype(auto) x4d = (i); // decltype(x4d) is int&
      |          ^~~~
      |          ----
p183.cpp:36:10: error: ISO C++ forbids declaration of 'parameter' with no type [-fpermissive]
p183.cpp:36:16: error: expected constructor, destructor, or type conversion before 'x4d'
   36 | decltype(auto) x4d = (i); // decltype(x4d) is int&
      |                ^~~
p183.cpp:37:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   37 | auto x5a = f(); // decltype(x5a) is int
      | ^~~~
      | ----
p183.cpp:37:6: error: 'x5a' does not name a type
   37 | auto x5a = f(); // decltype(x5a) is int
      |      ^~~
p183.cpp:38:10: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   38 | decltype(auto) x5d = f(); // decltype(x5d) is int&&
      |          ^~~~
      |          ----
p183.cpp:38:10: error: ISO C++ forbids declaration of 'parameter' with no type [-fpermissive]
p183.cpp:38:16: error: expected constructor, destructor, or type conversion before 'x5d'
   38 | decltype(auto) x5d = f(); // decltype(x5d) is int&&
      |                ^~~
p183.cpp:39:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   39 | auto x6a = { 1, 2 }; // decltype(x6a) is std::initializer_list<int>
      | ^~~~
      | ----
p183.cpp:39:6: error: 'x6a' does not name a type
   39 | auto x6a = { 1, 2 }; // decltype(x6a) is std::initializer_list<int>
      |      ^~~
p183.cpp:40:10: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   40 | decltype(auto) x6d = { 1, 2 }; // error: { 1, 2 } is not an expression
      |          ^~~~
      |          ----
p183.cpp:40:10: error: ISO C++ forbids declaration of 'parameter' with no type [-fpermissive]
p183.cpp:40:16: error: expected constructor, destructor, or type conversion before 'x6d'
   40 | decltype(auto) x6d = { 1, 2 }; // error: { 1, 2 } is not an expression
      |                ^~~
p183.cpp:41:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   41 | auto *x7a = &i; // decltype(x7a) is int*
      | ^~~~
      | ----
p183.cpp:41:7: error: ISO C++ forbids declaration of 'x7a' with no type [-fpermissive]
   41 | auto *x7a = &i; // decltype(x7a) is int*
      |       ^~~
p183.cpp:41:1: error: top-level declaration of 'x7a' specifies 'auto'
   41 | auto *x7a = &i; // decltype(x7a) is int*
      | ^~~~
p183.cpp:41:13: error: invalid conversion from 'const int*' to 'int*' [-fpermissive]
   41 | auto *x7a = &i; // decltype(x7a) is int*
      |             ^~
      |             |
      |             const int*
p183.cpp:42:10: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   42 | decltype(auto)*x7d = &i; // error: declared type is not plain decltype(auto)
      |          ^~~~
      |          ----
p183.cpp:42:10: error: ISO C++ forbids declaration of 'parameter' with no type [-fpermissive]
p183.cpp:42:15: error: expected constructor, destructor, or type conversion before '*' token
   42 | decltype(auto)*x7d = &i; // error: declared type is not plain decltype(auto)
      |               ^
rm: cannot remove 'p183g': No such file or directory

$ g++ p183.cpp -std=2b -o p183g -I. -Wall
p183.cpp:20:20: error: unable to deduce 'std::initializer_list<auto>' from '{1, 2.0e+0}'
   20 | auto x2 = { 1, 2.0 }; // error: cannot deduce element type
      |                    ^
p183.cpp:20:20: note:   deduced conflicting types for parameter 'auto' ('int' and 'double')
p183.cpp:21:15: error: direct-list-initialization of 'auto' requires exactly one element [-fpermissive]
   21 | auto x3{ 1, 2 }; // error: not a single element
      |               ^
p183.cpp:21:15: note: for deduction to 'std::initializer_list', use copy-list-initialization (i.e. add '=' before the '{')
p183.cpp:25:17: error: 'expr' was not declared in this scope
   25 | const auto &i = expr;
      |                 ^~~~
p183.cpp:40:29: error: could not convert '{1, 2}' from '<brace-enclosed initializer list>' to 'std::initializer_list<<brace-enclosed initializer list> >'
   40 | decltype(auto) x6d = { 1, 2 }; // error: { 1, 2 } is not an expression
      |                             ^
      |                             |
      |                             <brace-enclosed initializer list>
p183.cpp:42:16: error: 'decltype(auto)*' as type rather than plain 'decltype(auto)'
   42 | decltype(auto)*x7d = &i; // error: declared type is not plain decltype(auto)
      |                ^~~
```

## 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)

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


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 　20220625