JDK 10/11 RFR of JDK-8189146: Have use of "var" in 9 and earlier source versions issue a warning

[Maurizio Cimadamore]

Hi Joe,
I'm a bit unsure about having a retroactive warning in JDK 10 - e.g. I'm 
not sure how useful that would be, but I get the spirit of what you're 
getting at.

In terms of code, I think your warning should go inside the 
isRestrictedLocalVarTypeName(Name), which is called in a few places (not 
just the one you have touched). For instance, I don't think your patch 
will cover this case:

List<var> l = null;

So, the code for isRestrictedLocalVarTypeName(Name) is currently:

return allowLocalVariableTypeInference && name == names.var;

I think it should be updated to:

if (name == names.var) {
             if (allowLocalVariableTypeInference) {
                 return true;
             } else {
                 warning(pos, "var.not.allowed", name);
             }
         }
         return false;


Of course you need to enhance the method to take an extra position 
argument (pos).


Maurizio




On 09/01/18 18:28, joe darcy wrote:
> Hello,
>
> When the possibility of "_" being removed from the future set of valid 
> identifiers was determined, javac was updated to issue a warning to 
> that effect in JDK 8. Underscore was removed from the set of valid 
> identifiers as of JDK 9.
>
> With local variable type inference, the name "var" should be given 
> similar treatment in source versions 9 and earlier. Please review the 
> webrev which implements this:
>
>     http://cr.openjdk.java.net/~darcy/8189146.0/
>
> As written, the patch warns for a type or type variable named "var"; 
> it would be possible to expand the patch to warn about other uses of 
> "var" that would conflict with the JDK 10 feature. As a aid to 
> reviewing, the non-raw diagnostic output from 
> test/langtools/tools/javac/lvti/ParserTest.java under --release 9 is 
> listed below.
>
> IMO It would be preferable to issue this kind of warning in the 
> context of an -Xlint:future category (JDK-8189145), but until that 
> feature is developed, it is still better to issue the warning than to 
> not issue it.
>
> The webrev as presented is based off of JDK 11. Upon a successful code 
> review, I will explore getting the change into JDK 10 as an RFE and 
> make any needed adjustments to rebase the patch. If that is not 
> fruitful, I'll push the change to JDK 11.
>
> Thanks,
>
> -Joe
>
>
> ParserTest.java:14: warning: as of release 10, 'var' is a restricted 
> local variable type and cannot be used for type declarations
> class ParserTest<var> {
>                  ^
> ParserTest.java:16: warning: as of release 10, 'var' is a restricted 
> local variable type and cannot be used for type declarations
>         static class var { } //illegal
>                      ^
> ParserTest.java:20: warning: as of release 10, 'var' is a restricted 
> local variable type and cannot be used for type declarations
>         interface var { } //illegal
>                   ^
> ParserTest.java:24: warning: as of release 10, 'var' is a restricted 
> local variable type and cannot be used for type declarations
>         enum var { } //illegal
>              ^
> ParserTest.java:28: warning: as of release 10, 'var' is a restricted 
> local variable type and cannot be used for type declarations
>         @interface var { } //illegal
>                    ^
> ParserTest.java:36: warning: as of release 10, 'var' is a restricted 
> local variable type and cannot be used for type declarations
>     static class var extends RuntimeException { } //illegal
>                  ^
> 6 warnings
>