-preview=in might break code

[Adam D. Ruppe]

On Saturday, 3 October 2020 at 15:07:05 UTC, Paul Backus wrote:
> If the decision is ever made to make `-preview=in` the default, 
> the existing meaning of `in` should first be deprecated, and 
> eventually made into an error.

That has already been in progress over this last year, that's why 
`in` is now eligible for modifications.

The meaning has changed twice in the last year. One of them 
caused non-complying (already broken) code to fail to compile, so 
a second change came to ease up on it, but this was never going 
to be a permanent solution. The -preview switch's purpose is to 
see what final form it is going to take.

There probably will be a formal deprecation of it over the 
following year before the -preview is actually solidified. But 
the changes are already in flux.