Question

The following code snipplet tries to implement a ‘std::is_constructible<A, int>’:

#include <type_traits>

struct A { 
  // A(int);
};

template< typename T >
struct cstr_int
{
  static int mi;

  template< typename C >
  static typename std::enable_if< 
    std::is_same< decltype( T( mi ) ), T >::value, char >::type choose( C * );
  static long choose( ... );

  enum { value = ( sizeof( decltype( choose( & mi ) ) ) == 1 ) };
};

bool const b1 = cstr_int< A >::value;

Using g++ this works fine; using clang++ the following error is printed:

tmp/sfinae04.cc:14:29: error: no matching conversion for functional-style cast from 'int' to 'A'
    std::is_same< decltype( T( mi ) ), T >::value, char >::type choose( C * );
                            ^~~~~
tmp/sfinae04.cc:20:17: note: in instantiation of template class 'cstr_int<A>' requested here
bool const b1 = cstr_int< A >::value;
                ^
tmp/sfinae04.cc:3:8: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'int' to 'const A' for 1st argument
struct A { 
       ^
tmp/sfinae04.cc:3:8: note: candidate constructor (the implicit move constructor) not viable: no known conversion from 'int' to 'A' for 1st argument
struct A { 
       ^
tmp/sfinae04.cc:3:8: note: candidate constructor (the implicit default constructor) not viable: requires 0 arguments, but 1 was provided
1 error generated.

Version information: clang version 3.3 (trunk 172418) (llvm/trunk 172417), g++ (Debian 4.7.2-4) 4.7.2.

My question:
IMHO this should be a SFINAE. Do I miss something here or is there a problem with clang++?

(I know that there is is_constructible() but I’m not allowed to use it – and that is not the point of my question.)

Answer 1

Although it may look like a substitution-failure to you, it is actually a failure that is detected during template instantiation and thus a diagnosable error.

It is not a SFINAE context, because the enable_if does not depend on the template parameter(s) being deduced.

this works:

template< typename T >
struct cstr_int
{
  static int mi;

  template< typename C >
  static typename std::enable_if< 
    std::is_same< decltype( C( mi ) ), C >::value, char >::type choose( C * );
  static long choose( ... );

  enum { value = ( sizeof( decltype( choose( (T*)0 ) ) ) == 1 ) };
};