RFR: 8372635: Lambdas do not copy over SYNTHETIC flag for local variables

[Vicente Romero]

On Tue, 9 Dec 2025 15:04:42 GMT, Jan Lahoda <jlahoda at openjdk.org> wrote:

> Consider case like:
> 
> Object o = ...;
> 
> //synthetic variable for the instanceof's expression
> boolean b = o.toString() instanceof String str && str.isEmpty();
> 
> Runnable r = () -> {
>         //synthetic variable for the instanceof's expression
>         boolean b = o.toString() instanceof String str && str.isEmpty();
> };
> 
> 
> There are synthetic variables generated for `o.toString()`, to ensure the method is invoked only once for each `instanceof`. These variables are marked as synthetic in `TransPatterns` (and then consequently omitted in `LocalVariableTable`), but when the flag is lost for the variable in the lambda.
> 
> This PR proposes to keep the flag, as suggested in the bug.

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/LambdaToMethod.java line 1155:

> 1153:                     break;
> 1154:                 case LOCAL_VAR:
> 1155:                     ret = new VarSymbol(sym.flags() & (SYNTHETIC | FINAL), sym.name, sym.type, translatedSym);

this will effectively make synthetic all local variables, should we do this starting from source 27?, what about the parameter being generated below? should be it synthetic too?

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PR Review Comment: https://git.openjdk.org/jdk/pull/28724#discussion_r2604141496