-preview=in might break code

[Ola Fosheim Grøstad]

On Sunday, 4 October 2020 at 18:02:12 UTC, Johan wrote:
> My worry from reading this thread is that `-preview=in` is 
> making A platform/compiler dependent, to give users the feel 
> that they get some control over B. I very much hope I am wrong.

What is missing is the details of what the user must and must not 
do in order to ensure that all compliant compilers (all possible 
compilers that adhere to the spec) produce executables that 
generate the same output.

That shouldn't be guesswork, but explicit.