RFR: 8268312: Compilation error with nested generic functional interface [v5]

[Maurizio Cimadamore]

On Mon, 11 Jul 2022 03:18:39 GMT, Vicente Romero <vromero at openjdk.org> wrote:

>> Please review this PR, which is my proposal to fix an existing regression. This code:
>> 
>> 
>> import java.util.Optional;
>> 
>> class App {
>>     public static void main(String[] args) {
>>         Optional.of("").map(outer -> {
>>             Optional.of("")
>>                 .map(inner -> returnGeneric(outer))
>>                 .ifPresent(String::toString);
>>             return "";
>>         });
>>     }
>> 
>>     private static <RG> RG returnGeneric(RG generic) {
>>         return generic;
>>     }
>> }
>> 
>> is not accepted by javac but if the user passes the `-Xdiags:verbose` option then the code compiles. I tracked down the reason for this puzzling difference and I found that it is due to our diagnostic rewriters which can generate more detailed positions for error messages but in cases like the one above can trick the compiler to generate an error message too early. The code deciding if an error message should be deferred or not, depending on the position, is at `DeferredDiagnosticHandler::report`. We decide to do the rewriting if we are in diagnostics compact mode, this is why the error doesn't occur with the `-Xdiags:verbose` option. This fix will made some diagnostics to appear at a slightly different position, but won't make the compiler reject correct code. Comments?
>> 
>> TIA
>
> Vicente Romero has updated the pull request incrementally with one additional commit since the last revision:
> 
>   restoring the option for compressing diagnostics

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Resolve.java line 4177:

> 4175: 
> 4176:             Pair<Symbol, JCDiagnostic> c = errCandidate();
> 4177:             if (compactMethodDiags) {

I believe the issue is here - the old code made the rewriting dependent on the value of `compactMethodDiags`, which in turns looked at which diagnostic formatter was installed. I think we should either move this check in `Log` (when we decide if we need to rewrite), or don't attach a rewriter if the `compactMethodDiags` is not set. I think it would be preferable to keep existing diagnostics as they are (and maybe cleanup the behavior of `compactMethodDiags` later, as an orthogonal fix).

-------------

PR: https://git.openjdk.org/jdk/pull/5586