9.2.9.6.1 General [dcl.spec.auto.general] C++N4910:2022 (89) p180.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.

# 9.2.9.6.1 General [dcl.spec.auto.general] C++N4910:2022 (89) p180.cpp

## 算譜(source code)

```c++:p180.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="9.2.9.6.1 General [dcl.spec.auto.general] C++N4910:2022 (89) p180.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 x = 5; // OK, x has type int
const auto *v = &x, u = 6; // OK, v has type const int*, u has type const int
static auto y = 0.0; // OK, y has type double
auto int r; // error: auto is not a storage-class-specifier
auto f() -> int; // OK, f returns int
auto g() { return 0.0; } // OK, g returns double
auto h(); // OK, h’s return type will be deduced when it is defined
// [Example 2 :
auto x2 = 5, *y2 = &x2; // OK, auto is int
auto a = 5, b = { 1, 2 }; // error: different types for auto
// [Example 3 :
auto f3() { } // OK, return type is void
auto* g3() { } // error: cannot deduce auto* from void()
// [Example 4 :
auto n = n; // error: n’s initializer refers to n
auto f4();
void g4() { &f4; } // error: f’s return type is unknown
auto sum(int i) {
if (i == 1)
return i; // sum’s return type is int
else
return sum(i-1)+i; // OK, sum’s return type has been deduced
}
// [Example 5 :
template <class T> auto f(T t) { return t; } // return type deduced at instantiation time
typedef decltype(f(1)) fint_t; // instantiates f<int> to deduce return type
template<class T> auto f5(T* t) { return *t; }
void g5() { int (*p)(int*) = &f5; } // instantiates both fs to determine return types,
// chooses second
// [Example 6 :
auto f6();
auto f6() { return 42; } // return type is int
auto f6(); // OK
int f6(); // error: auto and int don’t match
decltype(auto) f6(); // error: auto and decltype(auto) don’t match
template <typename T> auto g6(T t) { return t; } // #1
template auto g6(int); // OK, return type is int
template char g6(char); // error: no matching template
template<> auto g6(double); // OK, forward declaration with unknown return type
template <class T> T g6(T t) { return t; } // OK, not functionally equivalent to #1
template char g6(char); // OK, now there is a matching template
template auto g6(float); // still matches #1
void h6() { return g6(42); } // error: ambiguous
template <typename T> struct A {
friend T frf(T);
};
auto frf(int i6) { return i6; } // not a friend of A<int>
extern int v6;
auto v6 = 17; // OK, redeclares v
struct S6 {
static int i6;
};
auto S6::i6 = 23; // OK
// [Example 7 :
template <typename T> auto f7(T t) { return t; }
extern template auto f7(int); // does not instantiate f<int>
int (*p)(int) = f7; // instantiates f<int> to determine its return type, but an explicit
// instantiation definition is still required somewhere in the program
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  p180
rm: cannot remove 'p180l': No such file or directory
rm: cannot remove 'p180g': No such file or directory
$ clang++ p180.cpp -std=03 -o p180l -I. -Wall
p180.cpp:19:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto x = 5; // OK, x has type int
^
p180.cpp:20:7: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
const auto *v = &x, u = 6; // OK, v has type const int*, u has type const int
      ^
p180.cpp:21:8: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
static auto y = 0.0; // OK, y has type double
       ^
p180.cpp:22:10: error: illegal storage class on file-scoped variable
auto int r; // error: auto is not a storage-class-specifier
         ^
p180.cpp:23:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto f() -> int; // OK, f returns int
^
p180.cpp:23:10: error: expected function body after function declarator
auto f() -> int; // OK, f returns int
         ^
p180.cpp:24:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto g() { return 0.0; } // OK, g returns double
^
p180.cpp:24:1: error: 'auto' not allowed in function return type
auto g() { return 0.0; } // OK, g returns double
^~~~
p180.cpp:25:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto h(); // OK, h’s return type will be deduced when it is defined
^
p180.cpp:25:1: error: 'auto' not allowed in function return type
auto h(); // OK, h’s return type will be deduced when it is defined
^~~~
p180.cpp:27:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto x2 = 5, *y2 = &x2; // OK, auto is int
^
p180.cpp:28:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto a = 5, b = { 1, 2 }; // error: different types for auto
^
p180.cpp:28:17: error: cannot deduce type of initializer list because std::initializer_list was not found; include <initializer_list>
auto a = 5, b = { 1, 2 }; // error: different types for auto
                ^
p180.cpp:30:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto f3() { } // OK, return type is void
^
p180.cpp:30:1: error: 'auto' not allowed in function return type
auto f3() { } // OK, return type is void
^~~~
p180.cpp:31:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto* g3() { } // error: cannot deduce auto* from void()
^
p180.cpp:31:1: error: 'auto' not allowed in function return type
auto* g3() { } // error: cannot deduce auto* from void()
^~~~
p180.cpp:33:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto n = n; // error: n’s initializer refers to n
^
p180.cpp:33:10: error: variable 'n' declared with deduced type 'auto' cannot appear in its own initializer
auto n = n; // error: n’s initializer refers to n
         ^
p180.cpp:34:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto f4();
^
p180.cpp:34:1: error: 'auto' not allowed in function return type
auto f4();
^~~~
p180.cpp:36:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto sum(int i) {
^
p180.cpp:36:1: error: 'auto' not allowed in function return type
auto sum(int i) {
^~~~
p180.cpp:43:20: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
template <class T> auto f(T t) { return t; } // return type deduced at instantiation time
                   ^
p180.cpp:43:20: error: 'auto' not allowed in function return type
template <class T> auto f(T t) { return t; } // return type deduced at instantiation time
                   ^~~~
p180.cpp:44:18: error: unknown type name 'f'
typedef decltype(f(1)) fint_t; // instantiates f<int> to deduce return type
                 ^
p180.cpp:44:20: error: expected ')'
typedef decltype(f(1)) fint_t; // instantiates f<int> to deduce return type
                   ^
p180.cpp:44:19: note: to match this '('
typedef decltype(f(1)) fint_t; // instantiates f<int> to deduce return type
                  ^
p180.cpp:44:24: error: expected function body after function declarator
typedef decltype(f(1)) fint_t; // instantiates f<int> to deduce return type
                       ^
p180.cpp:45:19: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
template<class T> auto f5(T* t) { return *t; }
                  ^
p180.cpp:45:19: error: 'auto' not allowed in function return type
template<class T> auto f5(T* t) { return *t; }
                  ^~~~
p180.cpp:46:31: error: address of overloaded function 'f5' does not match required type 'int (int *)'
void g5() { int (*p)(int*) = &f5; } // instantiates both fs to determine return types,
                              ^~
p180.cpp:49:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto f6();
^
p180.cpp:49:1: error: 'auto' not allowed in function return type
auto f6();
^~~~
p180.cpp:50:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto f6() { return 42; } // return type is int
^
p180.cpp:50:1: error: 'auto' not allowed in function return type
auto f6() { return 42; } // return type is int
^~~~
p180.cpp:51:1: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
auto f6(); // OK
^
p180.cpp:51:1: error: 'auto' not allowed in function return type
auto f6(); // OK
^~~~
p180.cpp:53:10: warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]
decltype(auto) f6(); // error: auto and decltype(auto) don’t match
         ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
19 warnings and 20 errors generated.
rm: cannot remove 'p180l': No such file or directory
$ clang++ p180.cpp -std=2b -o p180l -I. -Wall
p180.cpp:22:1: warning: 'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases [-Wauto-storage-class]
auto int r; // error: auto is not a storage-class-specifier
^~~~~
p180.cpp:22:10: error: illegal storage class on file-scoped variable
auto int r; // error: auto is not a storage-class-specifier
         ^
p180.cpp:28:1: error: 'auto' deduced as 'int' in declaration of 'a' and deduced as 'std::initializer_list<int>' in declaration of 'b'
auto a = 5, b = { 1, 2 }; // error: different types for auto
^        ~      ~~~~~~~~
p180.cpp:31:7: error: cannot deduce return type 'auto *' for function with no return statements
auto* g3() { } // error: cannot deduce auto* from void()
      ^
p180.cpp:33:10: error: variable 'n' declared with deduced type 'auto' cannot appear in its own initializer
auto n = n; // error: n’s initializer refers to n
         ^
p180.cpp:35:14: error: function 'f4' with deduced return type cannot be used before it is defined
void g4() { &f4; } // error: f’s return type is unknown
             ^
p180.cpp:34:6: note: 'f4' declared here
auto f4();
     ^
p180.cpp:46:19: warning: unused variable 'p' [-Wunused-variable]
void g5() { int (*p)(int*) = &f5; } // instantiates both fs to determine return types,
                  ^
p180.cpp:52:5: error: functions that differ only in their return type cannot be overloaded
int f6(); // error: auto and int don’t match
~~~ ^
p180.cpp:51:6: note: previous declaration is here
auto f6(); // OK
~~~~ ^
p180.cpp:53:16: error: functions that differ only in their return type cannot be overloaded
decltype(auto) f6(); // error: auto and decltype(auto) don’t match
~~~~~~~~~~~~~~ ^
p180.cpp:51:6: note: previous declaration is here
auto f6(); // OK
~~~~ ^
p180.cpp:56:15: error: explicit instantiation of 'g6' does not refer to a function template, variable template, member function, member class, or static data member
template char g6(char); // error: no matching template
              ^
p180.cpp:54:28: note: candidate template ignored: failed template argument deduction
template <typename T> auto g6(T t) { return t; } // #1
                           ^
p180.cpp:61:20: error: call to 'g6' is ambiguous
void h6() { return g6(42); } // error: ambiguous
                   ^~
p180.cpp:54:28: note: candidate function [with T = int]
template <typename T> auto g6(T t) { return t; } // #1
                           ^
p180.cpp:58:22: note: candidate function [with T = int]
template <class T> T g6(T t) { return t; } // OK, not functionally equivalent to #1
                     ^
2 warnings and 9 errors generated.

$ g++ p180.cpp -std=03 -o p180g -I. -Wall
p180.cpp:44:9: warning: identifier 'decltype' is a keyword in C++11 [-Wc++11-compat]
   44 | typedef decltype(f(1)) fint_t; // instantiates f<int> to deduce return type
      |         ^~~~~~~~
p180.cpp:19:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   19 | auto x = 5; // OK, x has type int
      | ^~~~
      | ----
p180.cpp:19:6: error: 'x' does not name a type
   19 | auto x = 5; // OK, x has type int
      |      ^
p180.cpp:20:7: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   20 | const auto *v = &x, u = 6; // OK, v has type const int*, u has type const int
      |       ^~~~
      |       ----
p180.cpp:20:13: error: ISO C++ forbids declaration of 'v' with no type [-fpermissive]
   20 | const auto *v = &x, u = 6; // OK, v has type const int*, u has type const int
      |             ^
p180.cpp:20:7: error: top-level declaration of 'v' specifies 'auto'
   20 | const auto *v = &x, u = 6; // OK, v has type const int*, u has type const int
      |       ^~~~
p180.cpp:20:18: error: 'x' was not declared in this scope
   20 | const auto *v = &x, u = 6; // OK, v has type const int*, u has type const int
      |                  ^
p180.cpp:21:8: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   21 | static auto y = 0.0; // OK, y has type double
      |        ^~~~
      |        ----
p180.cpp:21:13: error: 'y' does not name a type
   21 | static auto y = 0.0; // OK, y has type double
      |             ^
p180.cpp:22:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   22 | auto int r; // error: auto is not a storage-class-specifier
      | ^~~~
      | ----
p180.cpp:22:1: error: top-level declaration of 'r' specifies 'auto'
p180.cpp:23:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   23 | auto f() -> int; // OK, f returns int
      | ^~~~
      | ----
p180.cpp:23:6: error: ISO C++ forbids declaration of 'f' with no type [-fpermissive]
   23 | auto f() -> int; // OK, f returns int
      |      ^
p180.cpp:23:1: error: top-level declaration of 'f' specifies 'auto'
   23 | auto f() -> int; // OK, f returns int
      | ^~~~
p180.cpp:23:13: error: trailing return type only available with '-std=c++11' or '-std=gnu++11'
   23 | auto f() -> int; // OK, f returns int
      |             ^~~
p180.cpp:24:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   24 | auto g() { return 0.0; } // OK, g returns double
      | ^~~~
      | ----
p180.cpp:24:6: error: ISO C++ forbids declaration of 'g' with no type [-fpermissive]
   24 | auto g() { return 0.0; } // OK, g returns double
      |      ^
p180.cpp:24:1: error: top-level declaration of 'g' specifies 'auto'
   24 | auto g() { return 0.0; } // OK, g returns double
      | ^~~~
p180.cpp:24:1: error: storage class 'auto' invalid for function 'g'
p180.cpp:25:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   25 | auto h(); // OK, h’s return type will be deduced when it is defined
      | ^~~~
      | ----
p180.cpp:25:6: error: ISO C++ forbids declaration of 'h' with no type [-fpermissive]
   25 | auto h(); // OK, h’s return type will be deduced when it is defined
      |      ^
p180.cpp:25:1: error: top-level declaration of 'h' specifies 'auto'
   25 | auto h(); // OK, h’s return type will be deduced when it is defined
      | ^~~~
p180.cpp:25:1: error: storage class 'auto' invalid for function 'h'
p180.cpp:27:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   27 | auto x2 = 5, *y2 = &x2; // OK, auto is int
      | ^~~~
      | ----
p180.cpp:27:6: error: 'x2' does not name a type
   27 | auto x2 = 5, *y2 = &x2; // OK, auto is int
      |      ^~
p180.cpp:28:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   28 | auto a = 5, b = { 1, 2 }; // error: different types for auto
      | ^~~~
      | ----
p180.cpp:28:6: error: 'a' does not name a type
   28 | auto a = 5, b = { 1, 2 }; // error: different types for auto
      |      ^
p180.cpp:30:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   30 | auto f3() { } // OK, return type is void
      | ^~~~
      | ----
p180.cpp:30:6: error: ISO C++ forbids declaration of 'f3' with no type [-fpermissive]
   30 | auto f3() { } // OK, return type is void
      |      ^~
p180.cpp:30:1: error: top-level declaration of 'f3' specifies 'auto'
   30 | auto f3() { } // OK, return type is void
      | ^~~~
p180.cpp:30:1: error: storage class 'auto' invalid for function 'f3'
p180.cpp: In function 'int f3()':
p180.cpp:30:13: warning: no return statement in function returning non-void [-Wreturn-type]
   30 | auto f3() { } // OK, return type is void
      |             ^
p180.cpp: At global scope:
p180.cpp:31:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   31 | auto* g3() { } // error: cannot deduce auto* from void()
      | ^~~~
      | ----
p180.cpp:31:7: error: ISO C++ forbids declaration of 'g3' with no type [-fpermissive]
   31 | auto* g3() { } // error: cannot deduce auto* from void()
      |       ^~
p180.cpp:31:1: error: top-level declaration of 'g3' specifies 'auto'
   31 | auto* g3() { } // error: cannot deduce auto* from void()
      | ^~~~
p180.cpp:31:1: error: storage class 'auto' invalid for function 'g3'
p180.cpp: In function 'int* g3()':
p180.cpp:31:14: warning: no return statement in function returning non-void [-Wreturn-type]
   31 | auto* g3() { } // error: cannot deduce auto* from void()
      |              ^
p180.cpp: At global scope:
p180.cpp:33:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   33 | auto n = n; // error: n’s initializer refers to n
      | ^~~~
      | ----
p180.cpp:33:6: error: 'n' does not name a type
   33 | auto n = n; // error: n’s initializer refers to n
      |      ^
p180.cpp:34:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   34 | auto f4();
      | ^~~~
      | ----
p180.cpp:34:6: error: ISO C++ forbids declaration of 'f4' with no type [-fpermissive]
   34 | auto f4();
      |      ^~
p180.cpp:34:1: error: top-level declaration of 'f4' specifies 'auto'
   34 | auto f4();
      | ^~~~
p180.cpp:34:1: error: storage class 'auto' invalid for function 'f4'
p180.cpp: In function 'void g4()':
p180.cpp:35:13: warning: statement has no effect [-Wunused-value]
   35 | void g4() { &f4; } // error: f’s return type is unknown
      |             ^~~
p180.cpp: At global scope:
p180.cpp:36:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   36 | auto sum(int i) {
      | ^~~~
      | ----
p180.cpp:36:6: error: ISO C++ forbids declaration of 'sum' with no type [-fpermissive]
   36 | auto sum(int i) {
      |      ^~~
p180.cpp:36:1: error: top-level declaration of 'sum' specifies 'auto'
   36 | auto sum(int i) {
      | ^~~~
p180.cpp:36:1: error: storage class 'auto' invalid for function 'sum'
p180.cpp:43:20: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   43 | template <class T> auto f(T t) { return t; } // return type deduced at instantiation time
      |                    ^~~~
      |                    ----
p180.cpp:43:25: error: ISO C++ forbids declaration of 'f' with no type [-fpermissive]
   43 | template <class T> auto f(T t) { return t; } // return type deduced at instantiation time
      |                         ^
p180.cpp:43:20: error: top-level declaration of 'f' specifies 'auto'
   43 | template <class T> auto f(T t) { return t; } // return type deduced at instantiation time
      |                    ^~~~
p180.cpp:43:20: error: storage class 'auto' invalid for function 'f'
p180.cpp:44:9: error: ISO C++ forbids declaration of 'decltype' with no type [-fpermissive]
   44 | typedef decltype(f(1)) fint_t; // instantiates f<int> to deduce return type
      |         ^~~~~~~~
p180.cpp:44:9: error: typedef 'decltype' is initialized (use 'decltype' instead)
p180.cpp:45:19: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   45 | template<class T> auto f5(T* t) { return *t; }
      |                   ^~~~
      |                   ----
p180.cpp:45:24: error: ISO C++ forbids declaration of 'f5' with no type [-fpermissive]
   45 | template<class T> auto f5(T* t) { return *t; }
      |                        ^~
p180.cpp:45:19: error: top-level declaration of 'f5' specifies 'auto'
   45 | template<class T> auto f5(T* t) { return *t; }
      |                   ^~~~
p180.cpp:45:19: error: storage class 'auto' invalid for function 'f5'
p180.cpp: In function 'void g5()':
p180.cpp:46:19: warning: unused variable 'p' [-Wunused-variable]
   46 | void g5() { int (*p)(int*) = &f5; } // instantiates both fs to determine return types,
      |                   ^
p180.cpp: At global scope:
p180.cpp:49:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   49 | auto f6();
      | ^~~~
      | ----
p180.cpp:49:6: error: ISO C++ forbids declaration of 'f6' with no type [-fpermissive]
   49 | auto f6();
      |      ^~
p180.cpp:49:1: error: top-level declaration of 'f6' specifies 'auto'
   49 | auto f6();
      | ^~~~
p180.cpp:49:1: error: storage class 'auto' invalid for function 'f6'
p180.cpp:50:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   50 | auto f6() { return 42; } // return type is int
      | ^~~~
      | ----
p180.cpp:50:6: error: ISO C++ forbids declaration of 'f6' with no type [-fpermissive]
   50 | auto f6() { return 42; } // return type is int
      |      ^~
p180.cpp:50:1: error: top-level declaration of 'f6' specifies 'auto'
   50 | auto f6() { return 42; } // return type is int
      | ^~~~
p180.cpp:50:1: error: storage class 'auto' invalid for function 'f6'
p180.cpp:51:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   51 | auto f6(); // OK
      | ^~~~
      | ----
p180.cpp:51:6: error: ISO C++ forbids declaration of 'f6' with no type [-fpermissive]
   51 | auto f6(); // OK
      |      ^~
p180.cpp:51:1: error: top-level declaration of 'f6' specifies 'auto'
   51 | auto f6(); // OK
      | ^~~~
p180.cpp:51:1: error: storage class 'auto' invalid for function 'f6'
p180.cpp:53:10: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   53 | decltype(auto) f6(); // error: auto and decltype(auto) don’t match
      |          ^~~~
      |          ----
p180.cpp:53:10: error: ISO C++ forbids declaration of 'parameter' with no type [-fpermissive]
p180.cpp:53:16: error: expected constructor, destructor, or type conversion before 'f6'
   53 | decltype(auto) f6(); // error: auto and decltype(auto) don’t match
      |                ^~
p180.cpp:54:23: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   54 | template <typename T> auto g6(T t) { return t; } // #1
      |                       ^~~~
      |                       ----
p180.cpp:54:28: error: ISO C++ forbids declaration of 'g6' with no type [-fpermissive]
   54 | template <typename T> auto g6(T t) { return t; } // #1
      |                            ^~
p180.cpp:54:23: error: top-level declaration of 'g6' specifies 'auto'
   54 | template <typename T> auto g6(T t) { return t; } // #1
      |                       ^~~~
p180.cpp:54:23: error: storage class 'auto' invalid for function 'g6'
p180.cpp:55:10: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   55 | template auto g6(int); // OK, return type is int
      |          ^~~~
      |          ----
p180.cpp:55:15: error: ISO C++ forbids declaration of 'g6' with no type [-fpermissive]
   55 | template auto g6(int); // OK, return type is int
      |               ^~
p180.cpp:55:10: error: top-level declaration of 'g6' specifies 'auto'
   55 | template auto g6(int); // OK, return type is int
      |          ^~~~
p180.cpp:55:10: error: storage class 'auto' invalid for function 'g6'
p180.cpp:56:15: error: template-id 'g6<>' for 'char g6(char)' does not match any template declaration
   56 | template char g6(char); // error: no matching template
      |               ^~
p180.cpp:54:28: note: candidate is: 'template<class T> int g6(T)'
   54 | template <typename T> auto g6(T t) { return t; } // #1
      |                            ^~
p180.cpp:57:12: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   57 | template<> auto g6(double); // OK, forward declaration with unknown return type
      |            ^~~~
      |            ----
p180.cpp:57:17: error: ISO C++ forbids declaration of 'g6' with no type [-fpermissive]
   57 | template<> auto g6(double); // OK, forward declaration with unknown return type
      |                 ^~
p180.cpp:57:12: error: top-level declaration of 'g6' specifies 'auto'
   57 | template<> auto g6(double); // OK, forward declaration with unknown return type
      |            ^~~~
p180.cpp:57:12: error: storage class 'auto' invalid for function 'g6'
p180.cpp:57:12: error: explicit template specialization cannot have a storage class
p180.cpp:60:10: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   60 | template auto g6(float); // still matches #1
      |          ^~~~
      |          ----
p180.cpp:60:15: error: ISO C++ forbids declaration of 'g6' with no type [-fpermissive]
   60 | template auto g6(float); // still matches #1
      |               ^~
p180.cpp:60:10: error: top-level declaration of 'g6' specifies 'auto'
   60 | template auto g6(float); // still matches #1
      |          ^~~~
p180.cpp:60:10: error: storage class 'auto' invalid for function 'g6'
p180.cpp: In function 'void h6()':
p180.cpp:61:22: error: call of overloaded 'g6(int)' is ambiguous
   61 | void h6() { return g6(42); } // error: ambiguous
      |                    ~~^~~~
p180.cpp:54:28: note: candidate: 'int g6(T) [with T = int]'
   54 | template <typename T> auto g6(T t) { return t; } // #1
      |                            ^~
p180.cpp:58:22: note: candidate: 'T g6(T) [with T = int]'
   58 | template <class T> T g6(T t) { return t; } // OK, not functionally equivalent to #1
      |                      ^~
p180.cpp: At global scope:
p180.cpp:63:15: warning: friend declaration 'T frf(T)' declares a non-template function [-Wnon-template-friend]
   63 | friend T frf(T);
      |               ^
p180.cpp:63:15: note: (if this is not what you intended, make sure the function template has already been declared and add '<>' after the function name here)
p180.cpp:65:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   65 | auto frf(int i6) { return i6; } // not a friend of A<int>
      | ^~~~
      | ----
p180.cpp:65:6: error: ISO C++ forbids declaration of 'frf' with no type [-fpermissive]
   65 | auto frf(int i6) { return i6; } // not a friend of A<int>
      |      ^~~
p180.cpp:65:1: error: top-level declaration of 'frf' specifies 'auto'
   65 | auto frf(int i6) { return i6; } // not a friend of A<int>
      | ^~~~
p180.cpp:65:1: error: storage class 'auto' invalid for function 'frf'
p180.cpp:67:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   67 | auto v6 = 17; // OK, redeclares v
      | ^~~~
      | ----
p180.cpp:67:6: error: 'v6' does not name a type
   67 | auto v6 = 17; // OK, redeclares v
      |      ^~
p180.cpp:71:1: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   71 | auto S6::i6 = 23; // OK
      | ^~~~
      | ----
p180.cpp:71:10: error: 'i6' in 'struct S6' does not name a type
   71 | auto S6::i6 = 23; // OK
      |          ^~
p180.cpp:69:12: note: 'S6::i6' declared here
   69 | static int i6;
      |            ^~
p180.cpp:73:23: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   73 | template <typename T> auto f7(T t) { return t; }
      |                       ^~~~
      |                       ----
p180.cpp:73:28: error: ISO C++ forbids declaration of 'f7' with no type [-fpermissive]
   73 | template <typename T> auto f7(T t) { return t; }
      |                            ^~
p180.cpp:73:23: error: top-level declaration of 'f7' specifies 'auto'
   73 | template <typename T> auto f7(T t) { return t; }
      |                       ^~~~
p180.cpp:73:23: error: storage class 'auto' invalid for function 'f7'
p180.cpp:74:17: warning: 'auto' changes meaning in C++11; please remove it [-Wc++11-compat]
   74 | extern template auto f7(int); // does not instantiate f<int>
      |                 ^~~~
      |                 ----
p180.cpp:74:22: error: ISO C++ forbids declaration of 'f7' with no type [-fpermissive]
   74 | extern template auto f7(int); // does not instantiate f<int>
      |                      ^~
p180.cpp:74:17: error: top-level declaration of 'f7' specifies 'auto'
   74 | extern template auto f7(int); // does not instantiate f<int>
      |                 ^~~~
p180.cpp:74:17: error: storage class 'auto' invalid for function 'f7'
rm: cannot remove 'p180g': No such file or directory

$ g++ p180.cpp -std=2b -o p180g -I. -Wall
p180.cpp:22:1: error: two or more data types in declaration of 'r'
   22 | auto int r; // error: auto is not a storage-class-specifier
      | ^~~~
p180.cpp:28:1: error: inconsistent deduction for 'auto': 'int' and then 'std::initializer_list<int>'
   28 | auto a = 5, b = { 1, 2 }; // error: different types for auto
      | ^~~~
p180.cpp: In function 'auto* g3()':
p180.cpp:31:14: error: no return statements in function returning 'auto*'
   31 | auto* g3() { } // error: cannot deduce auto* from void()
      |              ^
p180.cpp:31:14: note: only plain 'auto' return type can be deduced to 'void'
p180.cpp:31:14: warning: no return statement in function returning non-void [-Wreturn-type]
p180.cpp: At global scope:
p180.cpp:33:10: error: use of 'n' before deduction of 'auto'
   33 | auto n = n; // error: n’s initializer refers to n
      |          ^
p180.cpp: In function 'void g4()':
p180.cpp:35:14: error: use of 'auto f4()' before deduction of 'auto'
   35 | void g4() { &f4; } // error: f’s return type is unknown
      |              ^~
p180.cpp: In function 'void g5()':
p180.cpp:46:19: warning: unused variable 'p' [-Wunused-variable]
   46 | void g5() { int (*p)(int*) = &f5; } // instantiates both fs to determine return types,
      |                   ^
p180.cpp: At global scope:
p180.cpp:52:5: error: ambiguating new declaration of 'int f6()'
   52 | int f6(); // error: auto and int don’t match
      |     ^~
p180.cpp:50:6: note: old declaration 'auto f6()'
   50 | auto f6() { return 42; } // return type is int
      |      ^~
p180.cpp:53:16: error: ambiguating new declaration of 'decltype(auto) f6()'
   53 | decltype(auto) f6(); // error: auto and decltype(auto) don’t match
      |                ^~
p180.cpp:50:6: note: old declaration 'auto f6()'
   50 | auto f6() { return 42; } // return type is int
      |      ^~
p180.cpp:56:15: error: template-id 'g6<>' for 'char g6(char)' does not match any template declaration
   56 | template char g6(char); // error: no matching template
      |               ^~
p180.cpp:54:28: note: candidate is: 'template<class T> auto g6(T)'
   54 | template <typename T> auto g6(T t) { return t; } // #1
      |                            ^~
p180.cpp: In function 'void h6()':
p180.cpp:61:22: error: call of overloaded 'g6(int)' is ambiguous
   61 | void h6() { return g6(42); } // error: ambiguous
      |                    ~~^~~~
p180.cpp:54:28: note: candidate: 'auto g6(T) [with T = int]'
   54 | template <typename T> auto g6(T t) { return t; } // #1
      |                            ^~
p180.cpp:58:22: note: candidate: 'T g6(T) [with T = int]'
   58 | template <class T> T g6(T t) { return t; } // OK, not functionally equivalent to #1
      |                      ^~
p180.cpp: At global scope:
p180.cpp:63:15: warning: friend declaration 'T frf(T)' declares a non-template function [-Wnon-template-friend]
   63 | friend T frf(T);
      |               ^
p180.cpp:63:15: note: (if this is not what you intended, make sure the function template has already been declared and add '<>' after the function name here)
p180.cpp:67:6: error: conflicting declaration 'auto v6'
   67 | auto v6 = 17; // OK, redeclares v
      |      ^~
p180.cpp:66:12: note: previous declaration as 'int v6'
   66 | extern int v6;
      |            ^~
p180.cpp:71:6: error: conflicting declaration 'auto S6::i6'
   71 | auto S6::i6 = 23; // OK
      |      ^~
p180.cpp:69:12: note: previous declaration as 'int S6::i6'
   69 | static int i6;
      |            ^~
p180.cpp:21:13: warning: 'y' defined but not used [-Wunused-variable]
   21 | static auto y = 0.0; // OK, y has type double
      |             ^

```

## 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