I've been thinking about the const situation again and I think these are groups of unrelated concepts:

    public, private, protected:  control visibility (if it's not visible,
it's not even an rvalue)

    const, mutable, non-const:  control assignability (rvalue vs. lvalue)

    volatile, non-volatile:  compiler should not make assumptions about it;
things are happening outside of its knowledge

    ref, non-ref:  control creation of new instances vs. simple restricted
"pointers" to instances

    user-defined attributes:  apply custom code, or can help influence
overloading and interface selection.

they can work on data, or on member functions.  ref and non-ref do not apply to member functions, only data such as parameters.

You need a few kinds of attributes:

    groups of mutually exclusive attributes, only one can apply at a time.

    solitary attributes, which default to off and you can switch on or off.
Need a way to turn them off!! "!const"?

Maybe "const" should be the default for parameters?  That might prevent so much mandatory use of const.   But then you'd definitely need a way to turn it off, for inout / out ref parameters.

Sean