1 // PR c++/87748
2 // { dg-do compile { target c++11 } }
3 
4 template <class T> T&& declval();
5 template <bool B, class T> struct enable_if;
6 template <class T> struct enable_if <true, T> { typedef T type; };
7 struct true_type { static const bool value = true; };
8 struct false_type { static const bool value = false; };
9 
10 struct string
11 {
12   string (const char *p);
13 };
14 
15 /// Template to map all type arguments to void, useful for SFINAE, see also WG21 N3911.
16 template<class...> using void_t = void;
17 
18 /// REQUIRES<value> - Simplified version of enable_if<> to use SFINAE in function templates.
19 template<bool value> using REQUIRES = typename ::enable_if<value, bool>::type;
20 
21 /// DerivesString<T> - Check if @a T is of type 'string'.
22 template<class T> struct DerivesString
23 {
24   static const int value = __is_same_as (T, string);
25 };
26 
27 /// Has__aida_visit__<T,Visitor> - Check if @a T provides a member template __aida_visit__<>(Visitor).
28 template<class, class, class = void> struct Has__aida_visit__ : false_type {};
29 template<class T, class V>
30 struct Has__aida_visit__<T, V, void_t< decltype (declval<T>().template __aida_visit__<V> (*(V*) 0)) >> : true_type {};
31 
32 struct Foo {
33   template<class V> void __accept__ (V*);
34 };
35 
36 /// Base template for Visitor classes, dispatches operator() to visit_<type>() methods.
37 template<class DerivedVisitor>
38 class VisitorDispatcher {
39   DerivedVisitor* derived () { return static_cast<DerivedVisitor*> (this); }
40 protected:
41   typedef const char* Name; ///< Member name argument type for visit() methods.
42 public:
43 
44   // dispatch for calls like: visitor (field, "field");
45 
46   template<class A,
47            REQUIRES< (!Has__aida_visit__<A, DerivedVisitor>::value) > = true>
48   void operator() (A &a, Name n)
49   { return derived()->visit_string (a, n); }
50 };
51 
52 class PspecVisitor : public VisitorDispatcher<PspecVisitor> {
53 public:
54   void visit_string (::string &a, Name name);
55 };
56 
57 int
58 main (int argc, char *argv[])
59 {
60 #ifdef WITHASSERT // define this to fix g++-8.1.1
61   static_assert (Has__aida_visit__<string, PspecVisitor>::value == false, "");
62 #endif
63   PspecVisitor v;
64   string str ("A string");
65   v (str, "some_string");
66   static_assert (Has__aida_visit__<string, PspecVisitor>::value == false, "");
67   return 0;
68 }
69 // g++ -std=gnu++14 -Wall -O2 aidavisit.cc && ./a.out
70