prototype mismatch with decltype and auto

Question

Consider the following class:

class MyClass
{
    int _id;
public:
    decltype(_id) getId();
};

decltype(MyClass::_id) MyClass::getId()
{
    return _id;
}

It compiles fine.

However when I make a template class out of it:

template <class T>
class MyClass
{
    int _id;
public:
    decltype(_id) getId();
};

template <class T>
decltype(MyClass<T>::_id) MyClass<T>::getId()
{
    return _id;
}

I get:

test.cpp:10:27: error: prototype for 'decltype (MyClass<T>::_id) MyClass<T>::getId()' does not match any in class 'MyClass<T>'
 decltype(MyClass<T>::_id) MyClass<T>::getId()                                                                                
                           ^
test.cpp:6:19: error: candidate is: decltype (((MyClass<T>*)(void)0)->MyClass<T>::_id) MyClass<T>::getId()
     decltype(_id) getId();
                   ^

Why is that?
Why the different types

• decltype (MyClass<T>::_id) MyClass<T>::getId()
• decltype (((MyClass<T>*)(void)0)->MyClass<T>::_id)

I could fix it by defining the body in the class:

template <class T>
class MyClass
{
    int _id;
public:
    decltype(_id) getId() { return _id; }
};

Trailing return type suffers a similar problem:

template <class T>
class MyClass
{
    int _id;
public:
    auto getId() -> decltype(_id);
};

template <class T>
auto MyClass<T>::getId() -> decltype(MyClass<T>::_id)
{
    return _id;
}

error:

test.cpp:10:6: error: prototype for 'decltype (MyClass<T>::_id) MyClass<T>::getId()' does not match any in class 'MyClass<T>'
 auto MyClass<T>::getId() -> decltype(MyClass<T>::_id)
      ^
test.cpp:6:10: error: candidate is: decltype (((MyClass<T>*)this)->MyClass<T>::_id) MyClass<T>::getId()
     auto getId() -> decltype(_id);
          ^

• decltype (MyClass<T>::_id) MyClass<T>::getId()
• decltype (((MyClass<T>*)this)->MyClass<T>::_id) MyClass<T>::getId()

g++ 5.3.0

Answer 1

According to the draft standard N4582 §5.1.1/p13 General [expr.prim.general] (Emphasis Mine):

An id-expression that denotes a non-static data member or non-static member function of a class can only be used:

(13.1) — as part of a class member access (5.2.5) in which the object expression refers to the member’s class⁶³ or a class derived from that class, or

(13.2) — to form a pointer to member (5.3.1), or

(13.3) — if that id-expression denotes a non-static data member and it appears in an unevaluated operand. [Example:

struct S {
int m;
};
int i = sizeof(S::m); // OK
int j = sizeof(S::m + 42); // OK

— end example ]

_{63) This also applies when the object expression is an implicit (*this) (9.3.1).}

Also from §7.1.6.2/p4 Simple type specifiers [dcl.type.simple](Emphasis Mine):

For an expression e, the type denoted by decltype(e) is defined as follows:

(4.1) — if e is an unparenthesized id-expression or an unparenthesized class member access (5.2.5), decltype(e) is the type of the entity named by e. If there is no such entity, or if e names a set of overloaded functions, the program is ill-formed;

(4.2) — otherwise, if e is an xvalue, decltype(e) is T&&, where T is the type of e;

(4.3) — otherwise, if e is an lvalue, decltype(e) is T&, where T is the type of e;

(4.4) — otherwise, decltype(e) is the type of e.

The operand of the decltype specifier is an unevaluated operand (Clause 5).

[Example:

const int&& foo();
int i;
struct A { double x; };
const A* a = new A();
decltype(foo()) x1 = 17; // type is const int&&
decltype(i) x2; // type is int
decltype(a->x) x3; // type is double
decltype((a->x)) x4 = x3; // type is const double&

— end example ] [ Note: The rules for determining types involving decltype(auto) are specified in 7.1.6.4. — end note ]

Consequently, since decltype is an unevaluated operand the code is legitimate and should compile.

One clean workaround would be to use decltype(auto):

template<typename T>
class MyClass {
  int _id;  
public:
  decltype(auto) getId();
};

template<typename T>
decltype(auto) MyClass<T>::getId() {
  return _id;
}

Above code is accepted by GCC/CLANG/VC++.

Answer 2

It seems like this is g++ bug.

I've tried your code in Visual Studio 2015:

Build: 1 succeeded, 0 failed, 0 up-to-date, 0 skipped

Edit: I found workaround:

#include <iostream>

template <class T>
class MyClass
{
    T _id = {0};
public:
    decltype(((MyClass<T>*)nullptr)->_id) getId();
};

template <class T>
decltype(((MyClass<T>*)nullptr)->_id) MyClass<T>::getId()
{
    return _id;
}

int main()
{
    MyClass<int> f;
    auto n = f.getId();

    std::cout << n << '\n'; // output: 0
}

Output:

0