Preview features and class file formats

[Alex Buckley]

On 4/21/2025 9:20 AM, Chen Liang wrote:
> That said, I wish to come back to the non SourceVersion/ 
> ClassFileFormatVersion-aware query methods - should they just return the 
> behavior when preview is enabled in the current release even when 
> preview is not enabled?

I thought we had agreed that if version-sensitive query methods can only 
be made aware of "25", not "25 with preview enabled", then their results 
should indicate the permanent behavior of 25 _regardless of whether the 
Java runtime was started with --enable-preview_.

> Coming back to the JEP 12 example of reserving _ as a keyword, the 
> SourceVersion.isIdentifier API will return false for _ in an 
> implementation of Java SE 8, regardless if the runtime has preview 
> features enabled or not, instead of only declaring that _ is not an 
> identifier when preview is enabled 

That's tricky because _ in 8 caused a warning but was technically legal. 
We knew there would be code compiled and run on 8 for many years, and 
did not wish to break it. _ only caused an error in 8 with 
--enable-preview. So, consistent with my cautious position above, 
SourceVersion.isIdentifier("_") would have returned true on an 8 runtime 
_regardless of whether the runtime was started with --enable-preview_.

Alex