Greetings,

I have a problem using decltype with some expression which involves
the private function call of some class.

I'm not sure it's a g++ bug or it's standard conformed, I'm using g+
+4.5.2/MinGW.

The following is a *contrived* example that illustrates the problem, I
hope you can get the idea:

--------------------------------------------------------------------------
// A non-member non-friend utility that needs to deduce the result
type of T::call(p).
template<class T, class P>
auto indirect_call(P p) ->  decltype(T::call(p))
{
     return T::call(p);
}

class A
{
     //
     // These ought to be used internally.
     //
     void private_fn() {}

     typedef void private_fn_result_type;

     struct UseDecltype
     {
         template<class P>
         static
         auto call(P p) ->  decltype(p->private_fn())
         {
             return p->private_fn();
         }
     };

     struct UseTypedef
     {
         template<class P>
         static
         A::private_fn_result_type call(P p)
         {
             return p->private_fn();
         }
     };

public:

     void use_decltype() // This NOT compiles
     {
         indirect_call<UseDecltype>(this);
     }

     void use_typedef() // This compiles
     {
         indirect_call<UseTypedef>(this);
     }
};
--------------------------------------------------------------------------
UseDecltype&  UseTypedef are both private of A, but UseXXX::call is
public-callable so we can use indirect_call<T>, but note that
indirect_call is an external utility which is neither member nor
friend.
In the case of UseTypedef, it works fine even
A::private_fn_result_type is private;
In the case of UseDecltype, it fails to compile because the compiler
complains that private_fn() which used in decltype is private of A...

The problem here:
Even though UseDecltype::call is public to indirect_call, but its
deduced type isn't, hmmm....

Could anybody shed some light on this matter?



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