-preview=in might break code

[Ola Fosheim Grøstad]

On Friday, 2 October 2020 at 19:37:43 UTC, Steven Schveighoffer 
wrote:
> On 10/2/20 3:20 PM, Ola Fosheim Grøstad wrote:
> That's just the thing. The -preview=in feature does not define 
> whether the parameter is pass by value or by reference. It 
> says, "up to the compiler".

Yes, I agree with you. The language spec should stick to the 
proper theoretical concept, e.g. pass by reference. It could also 
require no-aliasing of in-parameters and define aliasing as 
undefined behaviour that the compiler may or may not detect.

Disallowing aliasing can give better codegen (faster).

Then it could put in a footnote that some compilers optimize such 
and such as values in registers ( for those that read 
disassembled code ), but it should be no more than a footnote.

> I should be able to expect one behavior, and then if the 
> compiler can pass the parameter differently but I can't tell, 
> then that's fine. But this isn't that.

True.