[Bug c++/51710] New: decltype and SFINAE

["solodon at mail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51710

             Bug #: 51710
           Summary: decltype and SFINAE
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: solodon@mail.com


Hi,

I think the following code is well formed, but it fails to compile in g++ 4.6.1
and 4.5.2 (those that I had available). The code compiles in MS Visual C++
2010.

The problem is that decltype(foo(e1,e2)) in the result_type of enable_if below
fails the compilation instead of SFINAE.

#include <string>

/// Some meta-predicate on two types
template <typename E1, typename E2>
struct some_condition
{
    enum { value = false }; // Change to true to see it compile
};

/// Class can only be instantiated when some_condition<E1,E2>::value is true
template <typename E1, typename E2>
struct some_result_type
{
    /// Just an expression that will only compile when E1 and E2 are such
    int m[0-!some_condition<E1,E2>::value];            
};

/// This function doesn't have enable_if on it because 
/// I will only call it on the right types
template <typename E1, typename E2> 
some_result_type<E1,E2> foo(E1&& e1, E2&& e2)
{
    return some_result_type<E1,E2>();
} 

/// operator+ is overloaded for E1,E2 satisfying some_condition only
template <typename E1, typename E2> 
inline auto operator+(E1&& e1, E2&& e2) 
        -> typename std::enable_if<
               some_condition<E1,E2>::value, // We check condition
               decltype(foo(e1,e2)) // but this part fails the compilation
           >::type 
{ 
    return foo(std::forward<E1>(e1),std::forward<E2>(e2)); 
}

std::string my() { return "a"; }

void foo() 
{
    // The operator+ above should be disabled on arguments of these types
    // and regular addition of strings should be picked 
    std::string result = "aaaaa" + my(); 
}

int main() { foo(); }

When I compile this program I get:

In instantiation of 'some_result_type<const char (&)[6], basic_string<char> >':
exp2.cpp: 53:39:   instantiated from here
exp2.cpp: 26:42: error: size of array is negative

My system is Windows 7 Professional, with g++ 4.6.1 coming from MinGW release
from 2011-11-18 and g++ 4.5.2 coming from an earlier MinGW release. Command
line used: g++.exe -Wall -m32 -time -O2 -std=c++0x -o "exp2" "exp2.cpp"

Thanks,
Yuriy