Improving compiler messages for preview API

[Alex Buckley]

On 8/3/2019 1:48 PM, Jonathan Gibbons wrote:
> One topic for discussion is the interaction with the javac --release 
> option. For older releases, you can't use --enable-preview, so I guess 
> the behavior follows from that; for the current release, is there any 
> reason to allow or restrict the combination with --enable-preview?

You mean that JDK 13 would allow `javac --release 13 --enable-preview` 
but not `javac --release 12 --enable-preview`, on the grounds that when 
switch expressions previewed in SE 12, they could have been associated 
with an API which was potentially changed in SE 13?

Alex