-preview=in might break code

[Ola Fosheim Grøstad]

On Monday, 5 October 2020 at 00:31:33 UTC, kinke wrote:
> On Sunday, 4 October 2020 at 23:40:29 UTC, Ola Fosheim Grøstad 
> wrote:
>> It might help some if compilers would run unit tests 3 times 
>> with different 'in' implementations.
>>
>> 1 mixed value/ref
>> 2 value
>> 3 ref
>
> I was about to propose something like this (restricted to POD 
> types), possibly augmented by some indeterministic fuzzing. 
> Code ported to the new `in` semantics could then even show some 
> previously unintended aliasing issues; getting to the root of 
> the problem would probably still be non-trivial though.
>
> A compiler mode enforcing by-ref, coupled with some sort of 
> runtime sanitizer detecting invalid writes to live in-params, 
> would probably be a very valuable tool for validation & 
> troubleshooting.

You could do this: make it pass by value as the default, than add 
compiler switches that have increasingly aggressive optimizations 
up to assuming no aliasing.

That ought to be innocent enough.