-preview=in might break code

[Iain Buclaw]

On Monday, 5 October 2020 at 13:27:00 UTC, kinke wrote:
>
> Wrt. the concerns about differing ref/value decisions for PODs 
> across compilers/platforms and thus implementation-dependent 
> potential aliasing issues for lvalue args: a possible approach 
> could be leaving everything as-is ABI-wise, but have the 
> compiler create and pass a temporary in @safe callers if the 
> callee takes a ref, unless it can prove there's no way the arg 
> can be aliased. E.g., assuming x87 `real` for Win64:
>
> void callee(in real x); // e.g., by-ref for Win64, by-value for 
> Posix x86_64
>
> void safeCaller1(ref x) @safe
> {
>     callee(x); // x might be aliased by global state
>     // for Win64:    auto tmp = x, callee(tmp);
>     // Posix x86_64: by-value, so simply `callee(x)`
> }

So then `in` would come with its own semantic, that requires new 
code to handle, rather than piggy-backing off of `ref`?