13.9.2 Implicit instantiation [temp.inst]  C++N4910:2022 (234) p408.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.

# 13.9.2 Implicit instantiation [temp.inst]  C++N4910:2022 (234) p408.cpp

## 算譜(source code)

```c++:p408.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 = "13.9.2 Implicit instantiation [temp.inst]  C++N4910:2022 (234) p408.cpp";
// Debian clang version 14.0.5-++20220610033153+c12386ae247c-
// g++ (GCC) 12.1.0 Copyright (C) 2022 Free Software Foundation, Inc.
// Edited by Dr. Ogawa Kiyoshi. Compile procedure and results record.
// C++N4910:2022 Standard Working Draft on ISO/IEC 14882(0) sample code compile list
// https://qiita.com/kaizen_nagoya/items/fc957ddddd402004bb91

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cassert>
#include <coroutine>
#include <vector>
#include <complex>
#include <map>
#include <atomic>
#include <unordered_map>
#include <typeinfo>

using namespace std;

// Example 1
template<class T> class B { /* ... */
};
template<class T> class D : public B<T> { /* ... */
};
void f(void*);
void f(B<int>*);
void g(D<int>* p, D<char>* pp, D<double>* ppp) {
}
f(p);
B<char>* q = pp;
delete ppp;
// instantiation of D<int> required: call f(B<int>*)
// instantiation of D<char> required: convert D<char>* to B<char>*
// instantiation of D<double> required
// [Example 2:
template<class T> class X;
X<char> ch; // error: incomplete type X<char>
// [Example 3:
template<class T>
struct C {
    void f() {
        T x;
    }
    void g() = delete;
};
C<void> c; // OK, definition of C<void>::f is not instantiated at this point template<> void C<int>::g() { } // error: redefinition of C<int>::g
// [Example 4:
template<class T, class U>
struct Outer {
    template<class X, class Y> struct Inner;
    template<class Y> struct Inner<T, Y>;
    template<class Y> struct Inner<T, Y> { };
    template<class Y> struct Inner<U, Y> { };
};
Outer<int, int> outer;
// #1a
// #1b; OK, valid redeclaration of #1a // #2
// error at #2
//Outer<int, int>::Inner<int, Y> is redeclared at #1b. (It is not defined but noted as being associated with a definition in Outer<T, U>.) #2 is also a redeclaration of #1a. It is noted as associated with a definition, so it is an invalid redeclaration of the same partial specialization.
template<typename T> struct Friendly {
    template<typename U> friend int f(U) {
        return sizeof(T);
    }
};
Friendly<char> fc;
Friendly<float> ff; // error: produces second definition of f(U)
// [Example 5 :
template<class T> struct Z {
    void f();
    void g();
};
void h() {
    Z<int> a;
    Z<char>* p;
    Z<double>* q;
    a.f();
    p->g();
// instantiation of class Z<int> required
// instantiation of class Z<char> not required // instantiation of class Z<double> not required
// instantiation of Z<int>::f() required
// instantiation of class Z<char> required, and // instantiation of Z<char>::g() required
}
// Nothing in this example requires class Z<double>, Z<int>::g(), or Z<char>::f() to be implicitly instantiated.
// [Example 6:
template<typename T> constexpr int f() {
    return T::value;
}
template<bool B, typename T> void g(decltype(B ? f<T>() : 0));
template<bool B, typename T> void g(...);
template<bool B, typename T> void h(decltype(int{B ? f<T>() : 0}));
template<bool B, typename T> void h(...);
void x() {
    g<false, int>(0); // OK, B ? f<T>() : 0 is not potentially constant evaluated h<false, int>(0); // error, instantiates f<int> even though B evaluates to false and
}
f(a, b, c);
f(a, b);
f(a);
// no default argument instantiation
// default argument z = zdef(T()) instantiated // error: ydef is not declared
// list-initialization of int from int cannot be narrowing
}
// [Example 7:
template <class T> struct S {
    operator int();
};
void f(int);
void f(S<int>&);
void f(S<float>);
void g(S<int>& sr) {
    f(sr); // instantiation of S<int> allowed but not required
// instantiation of S<float> allowed but not required
};
// [Example 8:
template<class T> void f(T x, T y = ydef(T()), T z = zdef(T()));
class A {};
A zdef(A);
void g(A a, A b, A c) {
    f(a, b, c);
    f(a, b);
    f(a);
}
// [Example 9:
template<class T> class X {
    X<T>* p;
    X<T*> a;
// OK
// implicit generation of X<T> requires
// the implicit instantiation of X<T*> which requires
// the implicit instantiation of X<T**> which ...
};
// [Example 10:
template<typename T> concept C = sizeof(T) > 2;
template<typename T> concept D = C<T> && sizeof(T) > 4;
template<typename T> struct S {
    S() requires C<T> { }
    S() requires D<T> { }
};
S<char> s1;
S<char[8]> s2;
// #1 // #2
// error: no matching constructor // OK, calls #2
// When S<char> is instantiated, both constructors are part of the specialization. Their constraints are not satisfied, and they suppress the implicit declaration of a default constructor for S<char> (11.4.5.2), so there is no viable constructor for s1.
// [Example 11:
template<typename T> struct S1 {
    template<typename U>
    requires false
    struct Inner1; // ill-formed, no diagnostic required
};
template<typename T> struct S2 {
    template<typename U>
    requires (sizeof(T[-(int)sizeof(T)]) > 1)
    struct Inner2; // ill-formed, no diagnostic required
};
// The class S1<T>::Inner1 is ill-formed, no diagnostic required, because it has no valid specializations. S2 is ill-formed, no diagnostic required, since no substitution into the constraints of its Inner2 template would result in a valid expression.
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 p408
rm: cannot remove 'p408l': No such file or directory
rm: cannot remove 'p408g': No such file or directory
$ clang++ p408.cpp -std=c++03 -o p408l -I. -Wall
In file included from p408.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 \
 ^
p408.cpp:32:1: error: unknown type name 'f'
f(p);
^
p408.cpp:33:14: error: use of undeclared identifier 'pp'
B<char>* q = pp;
             ^
p408.cpp:34:1: error: expected unqualified-id
delete ppp;
^
p408.cpp:40:9: error: implicit instantiation of undefined template 'X<char>'
X<char> ch; // error: incomplete type X<char>
        ^
p408.cpp:39:25: note: template is declared here
template<class T> class X;
                        ^
p408.cpp:45:16: warning: deleted function definitions are a C++11 extension [-Wc++11-extensions]
    void g() = delete;
               ^
p408.cpp:54:28: error: class template partial specialization 'Inner<int, Y>' cannot be redeclared
  template<class Y> struct Inner<U, Y> { };
                           ^
p408.cpp:56:17: note: in instantiation of template class 'Outer<int, int>' requested here
Outer<int, int> outer;
                ^
p408.cpp:52:28: note: previous declaration of class template partial specialization 'Inner<int, type-parameter-0-0>' is here
  template<class Y> struct Inner<T, Y>;
                           ^
p408.cpp:62:40: error: redefinition of 'f'
       template<typename U> friend int f(U) { return sizeof(T); }
                                       ^
p408.cpp:65:17: note: in instantiation of template class 'Friendly<float>' requested here
Friendly<float> ff; // error: produces second definition of f(U)
                ^
p408.cpp:62:40: note: previous definition is here
       template<typename U> friend int f(U) { return sizeof(T); }
                                       ^
p408.cpp:74:14: warning: unused variable 'q' [-Wunused-variable]
  Z<double>* q;
             ^
p408.cpp:84:27: error: unknown type name 'constexpr'
     template<typename T> constexpr int f() { return T::value; }
                          ^
p408.cpp:85:40: warning: variable templates are a C++14 extension [-Wc++14-extensions]
     template<bool B, typename T> void g(decltype(B ? f<T>() : 0));
                                       ^
p408.cpp:85:40: error: variable has incomplete type 'void'
p408.cpp:85:57: error: 'T' does not refer to a value
     template<bool B, typename T> void g(decltype(B ? f<T>() : 0));
                                                        ^
p408.cpp:85:32: note: declared here
     template<bool B, typename T> void g(decltype(B ? f<T>() : 0));
                               ^
p408.cpp:85:60: error: expected expression
     template<bool B, typename T> void g(decltype(B ? f<T>() : 0));
                                                           ^
p408.cpp:87:40: warning: variable templates are a C++14 extension [-Wc++14-extensions]
     template<bool B, typename T> void h(decltype(int{B ? f<T>() : 0}));
                                       ^
p408.cpp:87:40: error: variable has incomplete type 'void'
p408.cpp:87:54: error: expected '(' for function-style cast or type construction
     template<bool B, typename T> void h(decltype(int{B ? f<T>() : 0}));
                                                  ~~~^
p408.cpp:92:1: error: C++ requires a type specifier for all declarations
f(a, b, c);
^
p408.cpp:92:3: error: use of undeclared identifier 'a'
f(a, b, c);
  ^
p408.cpp:92:6: error: use of undeclared identifier 'b'
f(a, b, c);
     ^
p408.cpp:93:3: error: unknown type name 'a'
f(a, b);
  ^
p408.cpp:93:6: error: unknown type name 'b'
f(a, b);
     ^
p408.cpp:93:1: error: C++ requires a type specifier for all declarations
f(a, b);
^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
4 warnings and 20 errors generated.
rm: cannot remove 'p408l': No such file or directory
$ clang++ p408.cpp -std=c++2b -o p408l -I. -Wall
p408.cpp:32:1: error: unknown type name 'f'
f(p);
^
p408.cpp:33:14: error: use of undeclared identifier 'pp'
B<char>* q = pp;
             ^
p408.cpp:34:1: error: expected unqualified-id
delete ppp;
^
p408.cpp:40:9: error: implicit instantiation of undefined template 'X<char>'
X<char> ch; // error: incomplete type X<char>
        ^
p408.cpp:39:25: note: template is declared here
template<class T> class X;
                        ^
p408.cpp:54:28: error: class template partial specialization 'Inner<int, Y>' cannot be redeclared
  template<class Y> struct Inner<U, Y> { };
                           ^
p408.cpp:56:17: note: in instantiation of template class 'Outer<int, int>' requested here
Outer<int, int> outer;
                ^
p408.cpp:52:28: note: previous declaration of class template partial specialization 'Inner<int, type-parameter-0-0>' is here
  template<class Y> struct Inner<T, Y>;
                           ^
p408.cpp:62:40: error: redefinition of 'f'
       template<typename U> friend int f(U) { return sizeof(T); }
                                       ^
p408.cpp:65:17: note: in instantiation of template class 'Friendly<float>' requested here
Friendly<float> ff; // error: produces second definition of f(U)
                ^
p408.cpp:62:40: note: previous definition is here
       template<typename U> friend int f(U) { return sizeof(T); }
                                       ^
p408.cpp:74:14: warning: unused variable 'q' [-Wunused-variable]
  Z<double>* q;
             ^
p408.cpp:92:1: error: C++ requires a type specifier for all declarations
f(a, b, c);
^
p408.cpp:92:3: error: use of undeclared identifier 'a'
f(a, b, c);
  ^
p408.cpp:92:6: error: use of undeclared identifier 'b'
f(a, b, c);
     ^
p408.cpp:93:3: error: unknown type name 'a'
f(a, b);
  ^
p408.cpp:93:6: error: unknown type name 'b'
f(a, b);
     ^
p408.cpp:93:1: error: C++ requires a type specifier for all declarations
f(a, b);
^
p408.cpp:94:1: error: unknown type name 'f'
f(a);
^
p408.cpp:98:1: error: extraneous closing brace ('}')
}
^
p408.cpp:111:37: error: use of undeclared identifier 'ydef'
template<class T> void f(T x, T y = ydef(T()), T z = zdef(T())); class A{};
                                    ^
p408.cpp:116:3: note: in instantiation of default function argument expression for 'f<A>' required here
  f(a);
  ^
p408.cpp:128:30: error: redefinition of 'C' as different kind of symbol
template<typename T> concept C = sizeof(T) > 2;
                             ^
p408.cpp:43:10: note: previous definition is here
  struct C {
         ^
p408.cpp:129:39: error: reference to 'C' is ambiguous
     template<typename T> concept D = C<T> && sizeof(T) > 4;
                                      ^
p408.cpp:43:10: note: candidate found by name lookup is 'C'
  struct C {
         ^
p408.cpp:128:30: note: candidate found by name lookup is 'C'
template<typename T> concept C = sizeof(T) > 2;
                             ^
p408.cpp:129:44: error: expected '(' for function-style cast or type construction
     template<typename T> concept D = C<T> && sizeof(T) > 4;
                                      ~~~~ ^
p408.cpp:130:29: error: redefinition of 'S'
template<typename T> struct S {
                            ^
p408.cpp:100:32: note: previous definition is here
     template <class T> struct S {
                               ^
1 warning and 19 errors generated.

$ g++ p408.cpp -std=c++03 -o p408g -I. -Wall
In file included from /usr/local/include/c++/12.1.0/atomic:38,
                 from p408.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 \
      |  ^~~~~
p408.cpp:84:27: warning: identifier 'constexpr' is a keyword in C++11 [-Wc++11-compat]
   84 |      template<typename T> constexpr int f() { return T::value; }
      |                           ^~~~~~~~~
p408.cpp:85:42: warning: identifier 'decltype' is a keyword in C++11 [-Wc++11-compat]
   85 |      template<bool B, typename T> void g(decltype(B ? f<T>() : 0));
      |                                          ^~~~~~~~
p408.cpp:32:2: error: expected constructor, destructor, or type conversion before '(' token
   32 | f(p);
      |  ^
p408.cpp:33:14: error: 'pp' was not declared in this scope
   33 | B<char>* q = pp;
      |              ^~
p408.cpp:34:1: error: expected unqualified-id before 'delete'
   34 | delete ppp;
      | ^~~~~~
p408.cpp:40:9: error: aggregate 'X<char> ch' has incomplete type and cannot be defined
   40 | X<char> ch; // error: incomplete type X<char>
      |         ^~
p408.cpp:45:16: warning: defaulted and deleted functions only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   45 |     void g() = delete;
      |                ^~~~~~
p408.cpp: In instantiation of 'struct Friendly<float>':
p408.cpp:65:17:   required from here
p408.cpp:62:40: error: redefinition of 'template<class U> int f(U)'
   62 |        template<typename U> friend int f(U) { return sizeof(T); }
      |                                        ^
p408.cpp:62:40: note: 'template<class U> int f(U)' previously declared here
p408.cpp: In function 'void h()':
p408.cpp:74:14: warning: unused variable 'q' [-Wunused-variable]
   74 |   Z<double>* q;
      |              ^
p408.cpp: At global scope:
p408.cpp:84:27: error: 'constexpr' does not name a type
   84 |      template<typename T> constexpr int f() { return T::value; }
      |                           ^~~~~~~~~
p408.cpp:84:27: note: C++11 'constexpr' only available with '-std=c++11' or '-std=gnu++11'
p408.cpp:85:40: error: variable or field 'g' declared void
   85 |      template<bool B, typename T> void g(decltype(B ? f<T>() : 0));
      |                                        ^
p408.cpp:85:58: error: expected primary-expression before '>' token
   85 |      template<bool B, typename T> void g(decltype(B ? f<T>() : 0));
      |                                                          ^
p408.cpp:85:60: error: expected primary-expression before ')' token
   85 |      template<bool B, typename T> void g(decltype(B ? f<T>() : 0));
      |                                                            ^
p408.cpp:85:42: error: there are no arguments to 'decltype' that depend on a template parameter, so a declaration of 'decltype' must be available [-fpermissive]
   85 |      template<bool B, typename T> void g(decltype(B ? f<T>() : 0));
      |                                          ^~~~~~~~
p408.cpp:85:42: note: (if you use '-fpermissive', G++ will accept your code, but allowing the use of an undeclared name is deprecated)
p408.cpp:87:40: error: variable or field 'h' declared void
   87 |      template<bool B, typename T> void h(decltype(int{B ? f<T>() : 0}));
      |                                        ^
p408.cpp:87:54: warning: extended initializer lists only available with '-std=c++11' or '-std=gnu++11' [-Wc++11-extensions]
   87 |      template<bool B, typename T> void h(decltype(int{B ? f<T>() : 0}));
      |                                                      ^
p408.cpp:87:51: error: expected primary-expression before 'int'
   87 |      template<bool B, typename T> void h(decltype(int{B ? f<T>() : 0}));
      |                                                   ^~~
p408.cpp:87:42: error: there are no arguments to 'decltype' that depend on a template parameter, so a declaration of 'decltype' must be available [-fpermissive]
   87 |      template<bool B, typename T> void h(decltype(int{B ? f<T>() : 0}));
      |                                          ^~~~~~~~
p408.cpp:92:2: error: expected constructor, destructor, or type conversion before '(' token
   92 | f(a, b, c);
      |  ^
p408.cpp:93:2: error: expected constructor, destructor, or type conversion before '(' token
   93 | f(a, b);
      |  ^
p408.cpp:94:2: error: expected constructor, destructor, or type conversion before '(' token
   94 | f(a);
      |  ^
p408.cpp:98:1: error: expected declaration before '}' token
   98 | }
      | ^
p408.cpp: In function 'void f(T, T, T) [with T = A]':
p408.cpp:111:41: error: 'ydef' was not declared in this scope; did you mean 'zdef'?
  111 | template<class T> void f(T x, T y = ydef(T()), T z = zdef(T())); class A{};
      |                                     ~~~~^~~~~
      |                                     zdef
p408.cpp:116:4: note:   when instantiating default argument for call to 'void f(T, T, T) [with T = A]'
  116 |   f(a);
      |   ~^~~
p408.cpp: At global scope:
p408.cpp:128:22: error: 'concept' does not name a type; did you mean 'const'?
  128 | template<typename T> concept C = sizeof(T) > 2;
      |                      ^~~~~~~
      |                      const
p408.cpp:128:22: note: 'concept' only available with '-std=c++20' or '-fconcepts'
p408.cpp:129:27: error: 'concept' does not name a type; did you mean 'const'?
  129 |      template<typename T> concept D = C<T> && sizeof(T) > 4;
      |                           ^~~~~~~
      |                           const
p408.cpp:129:27: note: 'concept' only available with '-std=c++20' or '-fconcepts'
p408.cpp:130:29: error: redefinition of 'struct S<T>'
  130 | template<typename T> struct S {
      |                             ^
p408.cpp:100:32: note: previous definition of 'struct S<T>'
  100 |      template <class T> struct S {
      |                                ^
p408.cpp:142:1: error: 'requires' does not name a type
  142 | requires false
      | ^~~~~~~~
p408.cpp:142:1: note: 'requires' only available with '-std=c++20' or '-fconcepts'
p408.cpp:147:11: error: expected identifier before 'sizeof'
  147 | requires (sizeof(T[-(int)sizeof(T)]) > 1)
      |           ^~~~~~
p408.cpp:147:11: error: expected ',' or '...' before 'sizeof'
p408.cpp:148:1: error: expected constructor, destructor, or type conversion before 'struct'
  148 | struct Inner2; // ill-formed, no diagnostic required
      | ^~~~~~
rm: cannot remove 'p408g': No such file or directory

$ g++ p408.cpp -std=c++2b -o p408g -I. -Wall
p408.cpp:32:2: error: expected constructor, destructor, or type conversion before '(' token
   32 | f(p);
      |  ^
p408.cpp:33:14: error: 'pp' was not declared in this scope
   33 | B<char>* q = pp;
      |              ^~
p408.cpp:34:1: error: expected unqualified-id before 'delete'
   34 | delete ppp;
      | ^~~~~~
p408.cpp:40:9: error: aggregate 'X<char> ch' has incomplete type and cannot be defined
   40 | X<char> ch; // error: incomplete type X<char>
      |         ^~
p408.cpp: In instantiation of 'struct Friendly<float>':
p408.cpp:65:17:   required from here
p408.cpp:62:40: error: redefinition of 'template<class U> int f(U)'
   62 |        template<typename U> friend int f(U) { return sizeof(T); }
      |                                        ^
p408.cpp:62:40: note: 'template<class U> int f(U)' previously declared here
p408.cpp: In function 'void h()':
p408.cpp:74:14: warning: unused variable 'q' [-Wunused-variable]
   74 |   Z<double>* q;
      |              ^
p408.cpp: At global scope:
p408.cpp:92:2: error: expected constructor, destructor, or type conversion before '(' token
   92 | f(a, b, c);
      |  ^
p408.cpp:93:2: error: expected constructor, destructor, or type conversion before '(' token
   93 | f(a, b);
      |  ^
p408.cpp:94:2: error: expected constructor, destructor, or type conversion before '(' token
   94 | f(a);
      |  ^
p408.cpp:98:1: error: expected declaration before '}' token
   98 | }
      | ^
p408.cpp: In function 'void f(T, T, T) [with T = A]':
p408.cpp:111:41: error: 'ydef' was not declared in this scope; did you mean 'zdef'?
  111 | template<class T> void f(T x, T y = ydef(T()), T z = zdef(T())); class A{};
      |                                     ~~~~^~~~~
      |                                     zdef
p408.cpp:116:4: note:   when instantiating default argument for call to 'void f(T, T, T) [with T = A]'
  116 |   f(a);
      |   ~^~~
p408.cpp: At global scope:
p408.cpp:128:30: error: conflicting declaration of template 'template<class T> concept C'
  128 | template<typename T> concept C = sizeof(T) > 2;
      |                              ^
p408.cpp:43:10: note: previous declaration 'template<class T> struct C'
   43 |   struct C {
      |          ^
p408.cpp:129:44: error: expected primary-expression before '&&' token
  129 |      template<typename T> concept D = C<T> && sizeof(T) > 4;
      |                                            ^~
p408.cpp:129:35: error: conflicting declaration of template 'template<class T> concept D'
  129 |      template<typename T> concept D = C<T> && sizeof(T) > 4;
      |                                   ^
p408.cpp:27:25: note: previous declaration 'template<class T> class D'
   27 | template<class T> class D : public B<T> { /* ... */ };
      |                         ^
p408.cpp:130:29: error: redefinition of 'struct S<T>'
  130 | template<typename T> struct S {
      |                             ^
p408.cpp:100:32: note: previous definition of 'struct S<T>'
  100 |      template <class T> struct S {
      |                                ^
```

## 検討事項(agenda)
コンパイルエラーを取るか、コンパイルエラーの理由を解説する。

<!-- リンクエラーの解説をする。clang++とg++の比較。意味のある出力。
コンパイルエラーを取るか、コンパイルエラーの理由を解説する。-->


# 参考資料(reference)

cpprefjp - C++日本語リファレンス

https://cpprefjp.github.io/

コンパイラの実装状況

https://cpprefjp.github.io/implementation-status.html


typedef は C++11 ではオワコン

https://qiita.com/Linda_pp/items/44a67c64c14cba00eef1

C99からC++14を駆け抜けるC++講座

https://qiita.com/yumetodo/items/e49a673afd9a3ecb81a8

## 自己参照(self reference)

コピペコンパイルエラーあるある

https://qiita.com/kaizen_nagoya/items/29c832de9d762aed2761

C++ Error Message Collection(1)does not name a type, 11 articles

https://qiita.com/kaizen_nagoya/items/acb9f1a9b5aad6df06bd

dockerにclang

https://qiita.com/kaizen_nagoya/items/8829ffeee397eda50e80

docker gnu(gcc/g++) and llvm(clang/clang++)

https://qiita.com/drafts/059874ea39c4de64c0f7

コンパイル用shell script C版(clangとgcc)とC++版(clang++とg++)

https://qiita.com/kaizen_nagoya/items/74220c0577a512c2d7da

Compare the contents of C++N4910:2022, C++N4741:2018 and C++N4606:2015

https://qiita.com/kaizen_nagoya/items/483246d40f98abff7ded

C++ sample list

https://qiita.com/kaizen_nagoya/items/54ad75b73825b7c04f86


clang++, g++コンパイルエラー方針の違いの例

https://qiita.com/kaizen_nagoya/items/ea6e5009fe126d270a82

astyle 使ってみた

https://qiita.com/kaizen_nagoya/items/3c9a87a781d53a1a23f3

C++N4606 Working Draft 2016, ISO/IEC 14882, C++ standardのコード断片をコンパイルするためにしていること
https://qiita.com/kaizen_nagoya/items/a8d7ee2f2e29e76c19c1

コンパイル用shell script C版(clangとgcc)とC++版(clang++とg++)
https://qiita.com/kaizen_nagoya/items/74220c0577a512c2d7da

Clang/Clang++(LLVM) gcc/g++(GNU) コンパイラ警告等比較
https://qiita.com/kaizen_nagoya/items/9a82b958cc3aeef0403f

C++2003とC++2017でコンパイルエラーになるならない事例集
https://qiita.com/kaizen_nagoya/items/a13ea3823441c430edff

Qiitaに投稿するCのStyle例(暫定)
https://qiita.com/kaizen_nagoya/items/946df1528a6a1ef2bc0d

cpprefjpのdecltypeをコンパイル試験
https://qiita.com/kaizen_nagoya/items/090909af702f0d5d8a67

MISRA C++ 5-0-16
https://qiita.com/kaizen_nagoya/items/7df2d4e05db724752a74

C++ Templates Part1 BASICS Chapter 3. Class Templates 3.2 Use of Class Template Stack stack1test.cpp
https://qiita.com/kaizen_nagoya/items/cd5fc49106fad5a4e9ed

ISO/IEC TS 17961:2013 C Secure Coding Rules(1) All list(to be confirmed)
https://qiita.com/kaizen_nagoya/items/54e056195c4f11b850a1

C言語（C++)に対する誤解、曲解、無理解、爽快。
https://qiita.com/kaizen_nagoya/items/3f3992c9722c1cee2e3a

C Puzzle Bookの有り難み5つ、C言語規格及びCコンパイラの特性を認識
https://qiita.com/kaizen_nagoya/items/d89a48c1536a02ecdec9

'wchar.h' file not found で困った clang++ macOS
https://qiita.com/kaizen_nagoya/items/de15cd46d657517fac11

Open POSIX Test Suiteの使い方を調べはじめました
https://qiita.com/kaizen_nagoya/items/644d5e407f5faf96e6dc

MISRA-C 2012 Referenceに掲載している文献の入手可能性を確認
https://qiita.com/kaizen_nagoya/items/96dc8b125e462d5575bb

どうやって MISRA Example Suiteをコンパイルするか
https://qiita.com/kaizen_nagoya/items/fbdbff5ff696e2ca7f00

MISRA C まとめ #include <misra_c.h>
https://qiita.com/kaizen_nagoya/items/f1a79a7cbd281607c7c9

「C++完全理解ガイド」の同意できること上位10
https://qiita.com/kaizen_nagoya/items/aa5744e0c4a8618c7671


 ＜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 　20220706