SFINAE与std :: begin的问题

Question

The following code works in Visual Studio 2013 but fails to compile with clang or gcc so I am guessing it is not valid c++.

#include <iterator>
#include <type_traits>
#include <vector>

template< typename T >
class SupportsStdBegin
{
  // std::false_type TestHasBegin( ... ) always exists.
  template< typename >
  static std::false_type TestSupportsBegin( ... );
  // std::true_type TestHasBegin( int ) only exists if std::begin( T() ) is a valid expression.
  template< typename U >
  static auto TestSupportsBegin( int ) -> decltype( std::begin( std::declval< T >() ), std::true_type{ } );

public:
  static const bool value = decltype( TestSupportsBegin< T >( 0 ) )::value;
};

static_assert( !SupportsStdBegin< int >::value, "'int' does not support std::begin()." );

const int TestArray[] = { 0, 1, 2, 3, 4, };
static_assert( SupportsStdBegin< decltype( TestArray ) >::value, "'const int TestArray[]' supports std::begin()." );

static_assert( SupportsStdBegin< std::vector< int > >::value, "'std::vector< int >' supports std::begin()." );

Can someone explain why SFINAE doesn't work as I'm expecting it to?

The line containing decltype( std::begin( std::declval< T >() ) fails for the static_assert for int type with the error:

error: no matching function for call to 'begin(int)'

I thought that this would mean SFINAE would fail and the implementation returning std::false_type would be used to set SupportsStdBegin< T >::value .

I understand there are alternate ways of writing this test but I would like to know why this particular way is not valid. I've successfully used it before to test if member functions exist.

Answer 1

For SFINAE to occur, the expression needs to be dependent on a template parameter in the immediate context of the template, ie the template parameter list.

The easy fix is to default U to T and use U in your dependent expression:

template <typename U=T>
static auto TestSupportsBegin( int ) 
    -> decltype( std::begin( std::declval< U >() ), std::true_type{ } );

Live Demo