Error with using decltype() in C++11 (creating opaque error message in gcc 4.7.0)

Question

with the following code (a boiled-down version of my original code)

#include 
#include 

template  class A;                   


        

Answer 1

There is an error with your code:

template
  auto diff(A const&y) const -> decltype(a-y.a)
  { return a-y.a; }

Here, A is a different type, so A cannot see it's a data member. Only A can see A::a.

Edit: that said, in your particular case, X and Y are both double, so I would naively expect that to compile. Note that in the best of cases, this construction should only compile when X and Y are the same, which may not be what you want.