Question

Updated below.
The following is the entire code I have in my main.cpp:

template<class T>
struct other_traits;

template<class T>
struct some_traits{
    typedef decltype(&T::operator()) Fty;
    typedef typename other_traits<Fty>::type type;
};

int main(){
}

But I get the following errors with Visual Studio 2010 while g++ compiles just fine:

src\main.cpp(9): error C2146: syntax error : missing ‘;’ before identifier ‘type’
–src\main.cpp(10) : see reference to class template instantiation ‘some_traits<T>‘ being compiled
src\main.cpp(9): error C2868: ‘some_traits<T>::type‘ : illegal syntax for using-declaration; expected qualified-name

(I like that last one, total wtf.)

Can I take that as a bug in VC10 or is there any good reason for the early instantiation? Or is it a bug with decltype that makes the compiler think that Fty is not a dependent name?

---

Update: I tried to cheat the compiler in thinking that Fty is a dependent name using a base class to inherit from:

template<class T>
struct other_traits;

template<class R, class C>
struct other_traits<R (C::*)()>{
    typedef R type;
};

template<class Fty>
struct base_traits{
    typedef typename other_traits<Fty>::type type;
};

template<class T>
struct some_traits
    : public base_traits<decltype(&T::operator())>
{};

But the compiler still tries to instantiate / compile everything on the spot, spewing these errors:

src\main.cpp(13): error C2039: 'type' : is not a member of 'other_traits<T>'
          with
          [
              T=
          ]
          src\main.cpp(19) : see reference to class template instantiation 'base_traits<Fty>' being compiled
          with
          [
              Fty=
          ]
          src\main.cpp(19) : see reference to class template instantiation 'some_traits<T>' being compiled
src\main.cpp(13): error C2146: syntax error : missing ';' before identifier 'type'
src\main.cpp(13): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
src\main.cpp(13): error C2602: 'base_traits<Fty>::type' is not a member of a base class of 'base_traits<Fty>'
          with
          [
              Fty=
          ]
          src\main.cpp(13) : see declaration of 'base_traits<Fty>::type'
          with
          [
              Fty=
          ]
src\main.cpp(13): error C2868: 'base_traits<Fty>::type' : illegal syntax for using-declaration; expected qualified-name
          with
          [
              Fty=
          ]

Note that the template parameters are empty. Any ideas?

Answer 1

It seems to be a Bug (if there is not special flag set as mentioned below).
Following is an excerpt from Oracle website for C++ templates:

7.2.2

The ISO C++ Standard permits
developers to write template classes
for which all members may not be
legal with a given template
argument. As long as the illegal
members are not instantiated, the
program is still well formed. The ISO
C++ Standard Library uses this
technique. However, the
-template=wholeclass option instantiates all members, and hence
cannot be used with such template
classes when instantiated with the
problematic template arguments.