c++ - decltype in class method declaration: error when used before "referenced" member is declared

Question

Consider the following code:

struct test {    
    auto func() -> decltype(data) {}  // ERROR

    int data;
};

int main() {
    test t;
    t.func();
}

It gives the following error:

main.cpp:2:29: error: 'data' was not declared in this scope
     auto func() -> decltype(data) {}

However, if I place data above func(), it gives out no error (live code):

struct test {    
    int data;

    auto func() -> decltype(data) {}
};

...

And so my question is, why is decltype not considering members declared after it (when decltype is used in a method declaration, not in the definition)? I also want to know if there are any change in this behavior in future iterations of the language standard.

---

^{Please note that I'm asking this because I was expecting decltype to behave differently. My coding convention is to place class data members below the class functions. Surely this different behavior would affect how I organize my class members. I would be very grateful if you can provide any workaround that would preserve my coding convention.}

Answer 1

The trailing return type is part of the member function declaration, which does not have access to data members or member functions declared after it, unlike the member function definition, which does. I am not aware of any change in this behaviour in C++14.

See 3.4.1-7 of the C++11 standard, Unqualified name look-up:

A name used in the definition of a class X outside of a member function body or nested class definition shall be declared in one of the following ways:

• before its use in class X or be a member of a base class of X (10.2), or...

(emphasis mine)