C++ Concepts simpliest example

concept.cpp

#include <list>
#include <iostream>
#include <algorithm>

#if defined(__cpp_concepts) && __cpp_concepts >= 201500 
template<typename T>
concept bool HasMemberNamedTestConcept = requires(T t) {
  t.test;
};

template<HasMemberNamedTestConcept T>
auto callConceptAsTemplateParameter(T t) {
  return t.test, t.test;
}
#endif

template<typename T>
#if defined(__cpp_concepts) && __cpp_concepts >= 201500
  requires HasMemberNamedTestConcept<T>
#endif
auto orUseRequireAfterTemplateParamList(T t) {
  return t.test;
}

template<typename T>
auto orUseRequireAfterDeclaration(T t)
#if defined(__cpp_concepts) && __cpp_concepts >= 201500
  requires HasMemberNamedTestConcept<T>
#endif
{
  return t.test;
}

struct NoTest {
};

struct HaveTest {
  int test;
};

int main()
{
#if defined(__cpp_concepts) && __cpp_concepts >= 201500
  std::cout <<  __cpp_concepts;
  auto x1 = callConceptAsTemplateParameter(NoTest{});
  auto y1 = callConceptAsTemplateParameter(HaveTest{});
#endif
  auto x2 = orUseRequireAfterTemplateParamList(NoTest{});
  auto y2 = orUseRequireAfterTemplateParamList(HaveTest{});
  auto x3 = orUseRequireAfterDeclaration(NoTest{});
  auto y3 = orUseRequireAfterDeclaration(HaveTest{});
  return 0;
}