The following program should compile, but current g++ from trunk compiles it.

Please read the comments in the code:

~=~

template<typename T>
struct do_typedef
{
  typedef T type;
};


template<typename T> struct is_function;

// Let's name this template partial specialization #1
template<typename T>
struct is_function< T ()>
{ 
};

template<typename T> struct is_member_func;

template<typename T, typename C>
struct is_member_func<T C::*>
{
  //[1] okay, the following line should not compile. Here is why.  
  //After the type substitution that happens at instantiation time,
  //the value of the argument that matches the T parameter is
  //'void () () const'.
  //In that context, the result of the substitution of T in the expression
  //do_typedef<T>::type _should_ be 'void () () const' as well.
  //So will be the argument of the is_function template.
  //'void () () const' is not compatible with 'T ()', so the  
  //partial template specialization #1 should be instantiated.
  //So we should get a compilation error.
  is_function<typename do_typedef<T>::type> t;
};


struct A;

// Should not compile because of the comment [1] above.
is_member_func<void (A::*) (void) const> t;
~=~

What happens is that during the type substitution of
do_typedef<T>::type, g++ removes the const qualifier from the 'void () () const' function. And that is a bug.

I'll attach below a patch that fixes it.