c++ default move assignment cannot access protected base member

Question

I'm having a piece of code that compiles with msvc having language extensions enabled, but not with language extensions disabled. It breaks down to this minimal example:

class A
{
    protected:
    A(const A&);
    A& operator=(const A&);
    A(A&&);
    A& operator=(A&&);
};
class B : public A
{
    public:
    B& operator=(B&&);
};
inline B& B::operator=(B&&) = default; // error C2248

With the error message being

C2248 'A::A': cannot access protected member declared in class 'A'

in line 14 where the move assignment operator is defined. Interestingly, the following code compiles just fine:

class A
{
    protected:
    A(const A&);
    A& operator=(const A&);
    A(A&&);
    A& operator=(A&&);
};
class B : public A
{
    public:
    B& operator=(B&&) = default;
};

Also interestingly, this only happens with the move-assignment-operator. The copy-assignment-operator, as well as the copy- and move-constructor (but not the move-assignment-operator) of B can be defined outside the class definition, inline, and default and the code will compile.

What am I doing wrong?

Answer 1

What am I doing wrong?

Nothing.

An explicitly defaulted definition would be exactly the same one a compiler would produce. In this case, it would move assign all bases and members. That requires the move assignment operators of those bases and members to be accessible to the one your are defaulting, of course. And yours is accessible, on account of being protected.

This is a MSVC++ bug. Try updating to a later version if you can.