Why can't I declare a reference to a mutable object? (“reference cannot be declared mutable”)

Question

Let\'s say we have a test.cpp as follows:

class A;

class B
{
    private:
        A mutable& _a;
};

Compilation:

Answer 1

According to the standard: [7.1.1 para 8]:

"The mutable specifier can be applied only to names of class data members (9.2) and cannot be applied to names declared const or static, and cannot be applied to reference members."

So it's just illegal.