替换失败有时是错误的吗？

Question

以下代码snipplet尝试实现'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;

使用g ++这很好用; 使用clang ++打印以下错误:

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.

版本信息:clang版本3.3(主干172418)(llvm/trunk 172417),g ++(Debian 4.7.2-4)4.7.2.

我的问题: 恕我直言,这应该是一个SFINAE.我在这里想念什么或者clang ++有问题吗？

(我知道有is_constructible()但我不允许使用它 - 这不是我的问题.)

Answer 1

虽然它可能看起来像是替换失败,但它实际上是在模板实例化期间检测到的故障,因此是可诊断的错误.

它不是 SFINAE上下文,因为enable_if它不依赖于推导的模板参数.

这工作:

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 ) };
};