Why can't I give a function's return type the scope storage class?

[Meta]

On Tuesday, 19 March 2019 at 02:03:12 UTC, Walter Bright wrote:
> On 3/18/2019 10:20 AM, Meta wrote:
>> So to be clear, you *can* add scope to the return type of a 
>> function, and it will propagate the shortest lifetime among 
>> any parameters marked with `return` or `return scope` to the 
>> return value?
>
> No. Adding scope as the storage class of the function means it 
> is attached to the 'this' parameter, if any.

Ah, I misinterpreted what you meant when you said "Scope on 
function return values comes from the scope of any arguments 
passed to the function marked as 'return scope'."

I pictured this:

scope T* doSomething(return scope T*);

Where the `scope` is attached to the T* return value, not to the 
function, but I think what you meant was this:

T* doSomething(return scope T*);

scope val = doSomething(someOtherVal);

Right?

> Any 'return scope' parameter to the function will constrict the 
> lifetime of the return value to that of the smallest lifetime 
> of such arguments.

Yes, I've got it now. Thank you.

>> Also, I'd like to echo the request that you document this 
>> stuff. What's already there is fairly useful, but obviously 
>> it's not complete.
>
> It can be improved. But my focus at the moment is getting 
> Phobos to compile with -dip1000.

Once I get a feel for DIP1000, I can hopefully help with that a 
bit.