// C++N4741 Committee Draft, Standard for Programming Language C++
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n741.pdf
const char* msg= "C++N4741(43)8.4.7.1 Simple requirements [expr.prim.req.simple]p97.cpp";
// Edited by Dr. Ogawa Kiyoshi. Compile procedure and results record.

#include <iostream>
#include <cstdlib>

using namespace std;

template<typename T> concept C =
requires (T a, T b) {
a + b; // C<T> is true if a + b is a valid expression
};

int main() {
  cout<< msg << endl;
  return EXIT_SUCCESS;
}

$ clang++ p95.cpp  -std=c++2a -Wall
p95.cpp:20:16: error: binary fold expression has unexpanded parameter packs in both operands
return (args + ... + args); // error: both operands contain unexpanded packs
        ~~~~   ^     ~~~~
1 error generated.

// C++N4741 Committee Draft, Standard for Programming Language C++
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n741.pdf
const char* msg= "C++N4741(43)8.4.7.1 Simple requirements [expr.prim.req.simple]p97.cpp";
// Edited by Dr. Ogawa Kiyoshi. Compile procedure and results record.

#include <iostream>
#include <cstdlib>
#include <type_traits>
#include <boost/concept_check.hpp>
#include <boost/concept_check/general.hpp>
#include <boost/concept/requires.hpp>
using namespace std;

template<typename T> concept C =
requires (T a, T b) {
  a + b; // C<T> is true if a + b is a valid expression
};

int main() {
  cout<< msg << endl;
  return EXIT_SUCCESS;
}

$ ../cplaf.sh p97 -fconcepts
$ clang++ p97.cpp  -std=c++03 -Wall
p97.cpp:14:22: error: unknown type name 'concept'
template<typename T> concept C =
                     ^
p97.cpp:14:30: warning: variable templates are a C++14 extension [-Wc++14-extensions]
template<typename T> concept C =
                             ^
p97.cpp:15:11: error: 'T' does not refer to a value
requires (T a, T b) {
          ^
p97.cpp:14:19: note: declared here
template<typename T> concept C =
                  ^
p97.cpp:15:16: error: 'T' does not refer to a value
requires (T a, T b) {
               ^
p97.cpp:14:19: note: declared here
template<typename T> concept C =
                  ^
1 warning and 3 errors generated.
$ clang++ p97.cpp  -std=c++2a -Wall
p97.cpp:14:22: error: expected unqualified-id
template<typename T> concept C =
                     ^
1 error generated.

$ g++-8 p97.cpp -fconcepts -std=c++03  -Wall
In file included from /usr/local/Cellar/gcc/8.1.0/include/c++/8.1.0/type_traits:35,
                 from p97.cpp:8:
/usr/local/Cellar/gcc/8.1.0/include/c++/8.1.0/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.
 #error This file requires compiler and library support \
  ^~~~~
p97.cpp:14:30: error: 'C' does not name a type
 template<typename T> concept C =
                              ^

$ g++-8 p97.cpp -fconcepts -std=c++2a  -Wall
p97.cpp:14:30: error: 'C' does not name a type
 template<typename T> concept C =
                              ^