Digamos que tenemos unatest.cpp como sigue

class A;

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

Compilacion

$> gcc test.cpp
test.cpp:6:20: error: reference ‘_a’ cannot be declared ‘mutable’ [-fpermissive]
gt; gcc test.cpp
test.cpp:6:20: error: reference ‘_a’ cannot be declared ‘mutable’ [-fpermissive]

My gcc:

$> gcc --version
gcc (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
gt; gcc --version
gcc (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

¿Por qué