Groovyc fails to correctly propagate classpath entries to javac when run under JDK 11

Repro Steps:

• unpack attached zip
• ensure CompileAntScript.xml references the version of groovy you have locally
• ensure JAVA_HOME is referencing JDK 11
• execute CompileAntScript.xml via Ant (ensure it is executed by JDK 11)

Actual Result: 

[compile] C:\dev\workspace\search0625_11\groovycIssue\src\org
[compile] \groovy\issue\TestCompile.java:3: error: package org.apache.commons.lang3 does not exist
[compile] import static org.apache.commons.lang3
[compile] .StringUtils.isEmpty;

Try running same ant script via JDK 8 and it will compile successfully.

Investigation Details:

Looking into groovy codebase it seems there is a bug/issue in how org.codehaus.groovy.tools.javac.JavacJavaCompiler.java accumulates classpath entries which should be passed to javac:

// append classpath if not already defined
if (!hadClasspath) {
    // add all classpaths that compilation unit sees
    List<String> paths = new ArrayList<String>(config.getClasspath());
    ClassLoader cl = parentClassLoader;
    while (cl != null) {
        if (cl instanceof URLClassLoader) {
            for (URL u : ((URLClassLoader) cl).getURLs()) {
                try {
                    paths.add(new File(u.toURI()).getPath());
                } catch (URISyntaxException e) {
                    // ignore it
                }
            }
        }
        cl = cl.getParent();
    }

It iterates over hierarchy of classloaders and assumes AppClassLoader is an instance of URLClassLoader. This assumption was correct in Java 8 but is not correct in Java 11 (https://blog.codefx.org/java/java-11-migration-guide/   - ‘Casting to URLClassLoader’ chapter). Thus most of jars are not passed to javac as classpath and compilation fails.