-preview=in might break code

[Joseph Rushton Wakeling]

On Saturday, 3 October 2020 at 16:56:06 UTC, Mathias LANG wrote:
> To put it differently: Why is `auto ref` acceptable but `in` is 
>  not ?

Aren't `auto ref` parameters more intuitive in their behaviour, 
though?  That the parameter will be passed by ref if it's an 
lvalue, and by value if it's an rvalue (in which case the only 
way it can mutate under your feet is if it wraps by reference 
some other app state)?

Unless I'm missing something, that's much more predictable and 
shouldn't suffer from implementation-dependent differences in the 
way the preview `in` design does.