DIP 1016--ref T accepts r-values--Formal Assessment

[kinke]

On Thursday, 24 January 2019 at 09:49:14 UTC, Manu wrote:
> We discussed and concluded that one mechanism to mitigate this 
> issue
> was already readily available, and it's just that 'out' gains a 
> much
> greater sense of identity (which is actually a positive 
> side-effect if
> you ask me!).
> You have a stronger motivation to use 'out' appropriately, 
> because it
> can issue compile errors if you accidentally supply an rvalue.

`out` with current semantics cannot be used as drop-in 
replacement for shared in-/output ref params, as `out` params are 
default-initialized on entry. Ignoring backwards compatibility 
for a second, I think getting rid of that would actually be 
beneficial (most args are probably already default-initialized by 
the callee in the line above the call...) - and I'd prefer an 
explicitly required `out` at the call site (C# style), to make 
the side effect clearly visible.

I'd have otherwise proposed a `@noRVal` param UDA, but redefining 
`out` is too tempting indeed. ;)