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#include <type_traits>
#include <iostream>
 
struct my_abstract_class { virtual int x() = 0; };
struct my_incomplete_class;
 
template<typename T, typename = void>
struct can_be_returned_from_function: std::false_type { };
 
template<typename T>
struct can_be_returned_from_function<T,
    typename std::enable_if<!std::is_abstract<T>::value,
    decltype(std::declval<T()>(), (void)0)>::type>
    : std::true_type { };
 
 
int main()
{
    ::std::cout
        << can_be_returned_from_function<int>::value << ::std::endl
        << can_be_returned_from_function<int[]>::value << ::std::endl
        << can_be_returned_from_function<int[2]>::value << ::std::endl
        << can_be_returned_from_function<int()>::value << ::std::endl
        << can_be_returned_from_function<my_abstract_class>::value << ::std::endl
        << can_be_returned_from_function<my_incomplete_class>::value << ::std::endl
        ;
    return 0;
}

In file included from prog.cpp:1:0:
/usr/include/c++/4.7/type_traits: In instantiation of ‘struct std::is_abstract<my_incomplete_class>’:
prog.cpp:25:62:   required from here
/usr/include/c++/4.7/type_traits:536:12: error: invalid use of incomplete type ‘struct my_incomplete_class’
prog.cpp:5:8: error: forward declaration of ‘struct my_incomplete_class’

Standard output is empty