The liabilities of binding rvalues to ref

[Jonathan M Davis]

On Friday, May 10, 2013 10:38:47 Manu wrote:
> What were the arguments again against ref const()? You're talking about
> making it clear that the function isn't planning on mutating the given
> rvalue...
> I understand that const is stronger than C++, but is it actually a
> deal-breaker? It's the most logical fit here.

Not being able to differentiate between when it's an rvalue and lvalue causes 
some problems, though Andrei understands those issues far better than I do. 
I've been able to come up with cases where it would be a problem if you could 
have ref on a local variable, but without that, I don't really understand 
where you end up with problems with const ref due to not being able to 
differentiate between rvalues and lvalues. It's easy to come up with cases with 
plain ref (e.g. the example in Ali's talk when a type's opAssign treated 
rvalues and lvalues differently, swapping guts with the rvalue and copying from 
the lvalue), but I'm not as clear on when it ends up being an issue with 
const. The fact that we don't allow ref on anything other than parameters and 
return types really simplifies things, so I'm not sure that the situation with 
const ref would be anywhere near as bad in D as Andrei thinks that it is in 
C++, but I don't know.

However, regardless of all that, the primary problem that I see with using 
const ref to indicate that you want to accept rvalues by ref is the fact that 
D's const is so restrictive, and there are types where const just plain 
doesn't work for them. As such, it strikes me as a bad idea to require const 
in order to accept rvalues by ref even if there are no other problems with it.

Now, you could end up with other weird problems if you used auto ref (assuming 
that we used auto ref for this) without const and the function _did_ mutate 
its argument (we already have that problem with auto ref and templates), but 
if you really want the extra protection and can afford it, you can always use 
const with it. The problem is the cases where you can't use const. We've been
trying very hard to make it so that const is completely optional precisely
because of how restrictive it is. Requiring it for this seems problematic to
me.

- Jonathan M Davis