Question

template<typename T> struct A {
    auto func() -> decltype(T::func()) {
        return T::func();
    }
};
class B : public A<B> {
    void func() {
    }
};

Seems pretty simple to me. But MSVC fails to compile.

visual studio 2010\projects\temp\temp\main.cpp(4): error C2039: 'func' : is not a member of 'B'
visual studio 2010\projects\temp\temp\main.cpp(8) : see declaration of 'B'
visual studio 2010\projects\temp\temp\main.cpp(8) : see reference to class template instantiation 'A<T>' being compiled
          with
          [
              T=B
          ]
visual studio 2010\projects\temp\temp\main.cpp(4): error C3861: 'func': identifier not found

Even though the compiler will happily accept calling the function. The below sample compiles fine.

template<typename T> struct A {
    void func() {
        return T::func();
    }
};
class B : public A<B> {
    void func() {
    }
};

I’ve got the same issue trying to use any types from the template argument.

template<typename T> struct A {
    typedef typename T::something something;
};
class B : public A<B> {
    typedef char something;
};

visual studio 2010\projects\temp\temp\main.cpp(4): error C2039: 'something' : is not a member of 'B'

Whereas class B clearly defines a type called “something”. The compiler is perfectly happy to call functions on an object of type T, T& or T*, but I can’t seem to access any types from T.

Answer 1

You are trying to use T::func before it was declared. That’s why the compiler shouts at you. Notice that when you derive from a class, the class is generated if it comes from a class template. And the implicit generation of the class (which is called implicit instantiation) necessiates the generation of declarations for all its members (so the compiler knows the sizeof value of the class, and can perform lookup into it).

So it also instantiates the declaration auto func() -> decltype(T::func()) and surely fails here.