-preview=in might break code

[Ola Fosheim Grøstad]

On Saturday, 3 October 2020 at 08:51:26 UTC, Daniel N wrote:
> You can freely mix any number of 'in' and 'out' parameters and 
> keep the optimized behavior.
>
> As soon as you add any 'ref' parameter 'in' will pass by value.

I don't know what would be the best mix. :-) But it is 
interesting to think about various options that could work.

There are many ways to make it a little better, but very 
difficult to come up with something that feels right.

For instance it is possible to use the type system to prove that 
in some cases aliasing is not possible simply because the types 
of objects that are reachable from parameter 1 are all different 
from the types reachable from parameter 2. Then you know that 
there can be no aliasing between the parameters. Unfortunately 
that excludes way too many valid cases where this does not hold.

And well, you also need to consider aliasing with globals that 
are accessed within the function.