add concept (gcc 6+)

[Dllieu]

    namespace
    {
        template <typename T>
        concept bool HasMemberStuff()
        {
            return requires( T t ) { t.stuff; };
        }

        void foo(const HasMemberStuff&) {}

        struct Foo{};
        struct FooWithStuff{int stuff;};
    } 

    foo(FooWithStuff());
    foo(Foo()); // constraints not satisfied void `foo(const HasMemberStuff&);`

[Dllieu]

template <typename T>
concept bool HasTradingPhase = requires(T t) {
  { t.tradingPhase };
};


struct Foo { int tradingPhase; };
struct Bar { int nottradingPhase; };


template <typename T> struct Wrapper;

template <typename T>
requires HasTradingPhase<T>
struct Wrapper<T> {
  int doSomething() { return t.tradingPhase; }

  T t;
};

template <typename T>
requires !HasTradingPhase<T>
struct Wrapper<T> {
  void somethingElse() {}
};


int main() {
Wrapper<Foo> foo;
foo.doSomething();

Wrapper<Bar> bar;
bar.somethingElse();
}