RFR: 8334488: Improve error for illegal early access from nested class

[Maurizio Cimadamore]

On Tue, 18 Jun 2024 16:12:55 GMT, Archie Cobbs <acobbs at openjdk.org> wrote:

> Consider this class:
> 
> class CtorPrologueBugTest {
>     int x;
>     CtorPrologueBugTest() {
>         this(new Object() {
>             { System.out.println(x); }
>         });
>     }
>     CtorPrologueBugTest(Object obj) {
>     }
> }
> 
> Prior to [JDK-8194743](https://bugs.openjdk.org/browse/JDK-8194743), it resulted in this error, generated by `Attr.java`:
> 
> CtorPrologueBugTest.java:5: error: cannot reference x before supertype constructor has been called
>             { System.out.println(x); }
>                                  ^
> 
> After [JDK-8194743](https://bugs.openjdk.org/browse/JDK-8194743), it resulted in this different error, generated later in the compilation process by `Lower.java`:
> 
> CtorPrologueBugTest.java:5: error: no enclosing instance of type CtorPrologueBugTest is in scope
>             { System.out.println(x); }
>                                  ^
> 
> There are two problems with this change in behavior:
> 
> 1. Such errors should be detected by `Attr.java`, not `Lower.java`, so that IDE's that only display errors generated by earlier compilation phases will show them.
> 1. The wording change in the error message is technically incorrect: `x` is in scope, it's just not yet accessible.
> 
> This change in behavior reflects an underlying logic bug, which is that `Attr::visitClassDef` is setting `ctorPrologue = false` on the containing class' environment, instead of the nested class' environment. As a result, when `Attr` descends into the nested class and then finds the `x` symbol in the containing class, it has "forgotten" that it is in an early construction context for `CtorPrologueBugTest` and fails to report the error at that point as it should.
> 
> This patch fixes this glitch to the error is detected earlier in the compilation process by `Attr` and is reported with the more correct verbiage.
> 
> Thanks to @mcimadamore for pointing out the mistake.

Looks good. While looking at this area earlier today, I'm also realizing how bad the error messages in this area are (not something this PR should be concerning with). E.g. "cannot reference xyz before superconstructor is called". *Which* superconstructor? Or, "cannot reference `this` from static context" - *which* `this`?

I think that, as we're here, we should probably try to improve the situation a little, at least for the next release.

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

Marked as reviewed by mcimadamore (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/19773#pullrequestreview-2126712501