ADL related GCC 4.7.2 issue with expression SFINAE

Question

Take the following code, which is characterized by

Reliance on ADL for a specific behavior (volume)
Using decltype for return type and relying on SFINAE to discard extra overloads

namespace Nature {
   struct Plant {};
   double volume(Plant){ return 3.14; }
}

namespace Industrial {
   struct Plant {};
   double volume(Plant) { return 100; }
}


namespace SoundEffects {
   // A workaround for GCC, but why?
   ////template<class T> void volume();

   template<class aSound>
   auto mix(aSound& s) -> decltype(volume(s)*0.1)
   {
      return volume(s)*.1;
   }

   struct Samples {
      Nature::Plant  np;
      Industrial::Plant ip;
   };


   inline double mix(const Samples& s) {
      return mix(s.np) + mix(s.ip);
   }
}

int main()
{
   SoundEffects::Samples s;
   assert( mix(s) ==  100*.1 + 3.14*.1 );
}

The code as presented (without the template<class T> void volume() line), VS 2012 and clang 3.5 compiles successfully, and runtime is as expected. However, GCC 4.7.2 says:

template-function-overload.cpp: In substitution of 'template<class aSound> decltype ((volume(s) * 1.0000000000000001e-1)) SoundEffects::mix(aSound&) [with aSound = SoundEffects::Samples]':
template-function-overload.cpp:46:4:   required from here
template-function-overload.cpp:23:9: error: 'volume' was not declared in this scope
template-function-overload.cpp:23:9: note: suggested alternatives:
template-function-overload.cpp:9:11: note:   'Nature::volume'
template-function-overload.cpp:14:11: note:   'Industrial::volume'

With the extra template volume line, all three compile and run fine.

So, there is clearly a compiler defect here. My question is, which compiler is the defective one? And which C++ standard is being violated?

David G · Accepted Answer · 2015-05-31T18:09:10.783

This was a bug which was fixed since GCC 4.8. Here's a simplified version of the code that gives the same error:

template<class T>
auto buzz(T x) -> decltype(foo(x));

void buzz(int);

int main() {
    buzz(5); // error: 'foo' was not declared in this scope
}

In mix(s), both overloads of SoundEffects::mix are compiled into the set of candidate overloads via ADL (SoundEffects is an associated namespace of SoundEffects::Sample). The function template overload is evaluated for viability. The error occurs because volume(s) cannot be resolved to a suitable overload either via pure unqualified lookup or ADL for Sample.

The reason this should pass is that when lookup fails a substitution failure should occur (since volume(s) is dependent) and the template should be rejected from overload resolution. This would leave mix(const Sample&) as the only viable overload to select. The fact that there is a hard error is clearly a sign that this GCC version has a faulty SFINAE implementation.

ADL related GCC 4.7.2 issue with expression SFINAE

1 Answers1