[Bug c++/57874] New: No SFINAE on ADL lookup failure

[gpderetta at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57874

            Bug ID: 57874
           Summary: No SFINAE on ADL lookup failure
           Product: gcc
           Version: 4.7.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gpderetta at gmail dot com

It seems that at least GCC 4.7 fails with an hard error on failure to lookup a
function via ADL in a sfinae context, if a function of that name is not visible
in scope. The following test fails to compile unless WORKAROUND is defined:

#ifdef WORKAROUND
struct unusable{};
void foo(unusable);
#endif

namespace NX
{
struct X {};
void foo(X){}
}

namespace NY
{
struct Y {};
}


template<class T>
auto ADLfoo(T&&x)  -> decltype((foo(T{}), short())); //decltype(foo(x));


char ADLfoo(...);

static_assert(sizeof(ADLfoo(NY::Y{})) == 1, "");
static_assert(sizeof(ADLfoo(NX::X{})) == 2, "");

int main(){}

# g++ --std=c++11 -C adl.cc
adl.cc: In substitution of ‘template<class T> decltype ((foo(T{}), short
int())) ADLfoo(T&&) [with T = NY::Y]’:
adl.cc:24:36:   required from here
adl.cc:19:6: error: ‘foo’ was not declared in this scope
adl.cc:19:6: note: suggested alternative:
adl.cc:9:6: note:   ‘NX::foo’

Clang 3.2 compiles this fine.