Reference to the this pointer: GCC vs clang

Question

This is a follow-up of these questions.

Consider the following code:

struct A {
private:
    A* const& this_ref{this};
};

int main() {
    A a{};
    (void)a;
}

If compiled with the -Wextra, both GCC v6.2 and clang v3.9 show a warning.

Anyway, with the slightly modified version shown below they behave differently:

struct A {
    A* const& this_ref{this};
};

int main() {
    A a{};
    (void)a;
}

In this case GCC doesn't give any warning, clang gives the same warning as returned in the previous example.

The warnings are almost the identical.
It follows the one from clang:

3 : warning: binding reference member 'this_ref' to a temporary value [-Wdangling-field]

Which compiler is right?

I would say that GCC is wrong in this case and I were opening an issue, but maybe it's the opposite because of an arcane corner case of the language.

Answer 1

The reason for this warning is IMO this excerpt from standard (12.2.5):

A temporary bound to a reference member in a constructor’s ctor-initializer (12.6.2) persists until the constructor exits.

and since the keyword this is a prvalue expression, during this_ref initialization a temporary will be created and this_ref is bound to that temporary.

But I have doubt whether your reference is actually initialized in ctor-initializer.

If you write:

struct A {
private:
    const int& rr = 1+1;
};

then you will reproduce the exact same problem with gcc, removing private will also remove this warning.

From what I know this pointer might be used in the body of the non-static member function, I have never read that it could be used as argument during default member initialization.