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

As I have figured out by now (see also the edits to the question) what the problem was. It turns out that all answers (sofar) didn't solve the (full) problem.

Essentially, the evaluation of the decltype expression for the global function required eventually the private member A::a (through its occurrence in the decltype expression of A::diff()). This opens the next question about what the standard says about this.