ECJ batch compiler only loads multi-release classes with exact --release match

[Marcono1234]

This is a clone of https://bugs.eclipse.org/bugs/show_bug.cgi?id=577790, as suggested in #2773 (comment)

Version

ecj-3.38.0.jar from Maven Central

(and earlier versions, see Bugzilla report)

Description

Libraries which want to target Java 8 (respectively Android) as lowest version, but also want to provide a module descriptor commonly use multi-release JARs for this, e.g. META-INF/versions/9/module-info.class. This is also explicitly mentioned in the JAR specification:

A module descriptor may be present under a versioned area but not present under the top-level directory.

The problem is that the ECJ batch compiler when used with --release N only considers META-INF/versions/N, but not any directories < N of libraries in the module path.

This differs from javac behavior, from Java runtime behavior (as required by the JAR specification) and also from Eclipse JDT behavior within the IDE (that has a different problem though, see #2495).

This report here is only about --release; currently the ECJ batch compiler does not read multi-release classes at all when using -target, see #2774.

There was a pull request for this, but it has been abandoned: #61

Reproduction steps

1. Download the attached multi-release-test.zip and unzip it
2. Download ECJ, for example from https://search.maven.org/artifact/org.eclipse.jdt/ecj
3. Run ECJ with --release 11

   java -jar ./ecj.jar --release 11 --module-path ./multi-release.jar ./module-info.java
   

   ❌ Bug: It cannot find the referenced module
4. Run ECJ with --release 10 (which matches exactly the META-INF/versions/10 in the JAR)

   java -jar ./ecj.jar --release 10 --module-path ./multi-release.jar ./module-info.java
   

   ℹ️ As expected: Compilation succeeds