c++ - Ambiguous call in custom member detector -

i working in own implementation of member detector improve programming skills. following code compile g++ clang++ reject code, error :

error: call 'check' ambiguous main.cpp:19:17: note: candidate function [with u = has_member::hasit]

static char check( decltype(u::i)* );

main.cpp:22:16: note: candidate function [with u = has_member::hasit]

static int check(u*);

here's code of class

template<typename t> struct has_member  {      struct fallback     {         int i;     };      struct hasit : fallback, t     {};      template<class u>     static char check( decltype(u::i)* );       template<typename u>     static int check(u*);          static const bool value = sizeof(check<hasit>( nullptr ) ) == sizeof(int); };  class test { public:  };  int main() {     auto v  = has_member<test>::value;      std::cout << std::boolalpha << v; } 

live example here

the question : code valid ? if why g++ accepts ?

the code should valid. have 2 viable candidates:

template <class u> static char check( decltype(u::i)* ); // --> int*  template <class u> static int check( u* ); // --> hasit* 

nullptr convertible both pointer types, neither conversion better other, both candidates function templates. former more specialized latter, should preferred. clang bug.

a simple workaround works both compilers change second overload instead be:

template<typename u> static int check(...);    

since preferred on ellipses, still remains fallback option, without having rely on template partial ordering rules.

furthermore, since we're in c++11, can use return types directly:

template<class u> static std::false_type check( decltype(u::i)* );   template<typename u> static std::true_type check(...);  using type = decltype(check<hasit>(nullptr)); static constexpr bool value = type::value;