return const proposals

[ore-sama]

I wanted to use const system, but the kitchen sink syndrome SUDDENLY killed me :(
There is return storage class attribute

const(char[]) capitalize(return const(char[]) s) { ... }

but it's not very intuitive: surprisingly return type is NOT what it looks like and one have to read the entire signature to figure out that the return type is not really const.
Rather than that we want here one parameter with *optional* constness and return type *inheriting* this constness and these tend to be deemed as *two* different, though related, operations. And as Bill Baxter pointed out ( http://d.puremagic.com/issues/show_bug.cgi?id=2094#c9 ) we would like to extend it to invariant inheritance. So this gets a little more complicated than mere attribute. So a few proposals from me.

Case 1. Straight-forward approach. Huh? and bang! notation.
const!(str)(char[]) capitalize(const?(char[]) str) { ... }
So the second huh?const declares mimic const type and the first bang!const extracts constness from parameter and applies it to the return type.
* return type explicitly indicates what it does.
* optional const and inheriting const are different independent entities.

Case 2. We need only const-extraction operator... pseudo-operator?
const!(str)(char[]) capitalize(const!(str)(char[]) str) { ... }
Constness of parameter gets equal to constness of argument!
* two operations reduced to one
* more context-independent.

Case 3. Quantum const.
const?(char[]) capitalize(const?(char[]) s) { ... }
Like huh?const, but all of them from the same signature enter entangled state (with indefinite constness) and trigger simultaneously (constness gets determined) on function call (interaction with with the world). This rationale came from quantum mechanics :)))
* possibly a third flavor of const
* extension on invariant can be done also in terms of quantum mechanics: just say const? can take 3 values: mutable, const and invariant, but its value is indefinite until function call.
* D will be as complicated as quantum mechanics is. Joke.

P.S. this can be easily extended for member functions
const(char[]) capitalize() return const
const!(this)(char[]) capitalize()  //no huh?const because this is already huh?const
const!(this)(char[]) capitalize()  //the same for operator form
const?(char[]) capitalize() const?  //or we can also deem this as quantum-consted (of indefinite constness).