ADDITIONAL SYSTEM INFORMATION :
OS: Arch Linux
Tested with javadoc 23, 17 (and javadoc 8, only works as expected with javadoc 8)

java-23/java -version:
openjdk version "23" 2024-09-17
OpenJDK Runtime Environment (build 23)
OpenJDK 64-Bit Server VM (build 23, mixed mode, sharing)

java-17/java -version
openjdk version "17.0.13" 2024-10-15
OpenJDK Runtime Environment (build 17.0.13+11)
OpenJDK 64-Bit Server VM (build 17.0.13+11, mixed mode, sharing)

A DESCRIPTION OF THE PROBLEM :
if javadoc is invoked with the -subpackages option and the source is set to 8, the program fails with an AssertionError (see the ActualResult section for a trace).


REGRESSION : Last worked in version 8

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
javadoc -source 8 -subpackages test
# also fails with javadoc -source 8 -subpackages test test.pkg
# also fails with javadoc -source 8 -subpackages test.pkg
# also fails if -d and -sourcepath is used


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Something like:

Loading source files for package test.pkg...
Constructing Javadoc information...
Building index for all the packages and classes...
Standard Doclet version 23
Building tree for all the packages and classes...
Generating ./test/pkg/Test.html...
./test/pkg/Test.java:2: warning: no comment
public class Test {
       ^
./test/pkg/Test.java:2: warning: use of default constructor, which does not provide a comment
public class Test {
       ^
Generating ./test/pkg/package-summary.html...
Generating ./test/pkg/package-tree.html...
Generating ./overview-tree.html...
Generating ./allclasses-index.html...
Building index for all classes...
Generating ./allpackages-index.html...
Generating ./index-all.html...
Generating ./search.html...
Generating ./index.html...
Generating ./help-doc.html...
2 warnings
ACTUAL -
Loading source files for package test...
Constructing Javadoc information...
error: fatal error encountered: java.lang.AssertionError
error: Please file a bug against the javadoc tool via the Java bug reporting page
  (https://bugreport.java.com) after checking the Bug Database (https://bugs.java.com)
  for duplicates. Include error messages and the following diagnostic in your report. Thank you.
java.lang.AssertionError
        at jdk.compiler/com.sun.tools.javac.util.Assert.error(Assert.java:155)
        at jdk.compiler/com.sun.tools.javac.util.Assert.checkNonNull(Assert.java:62)
        at jdk.compiler/com.sun.tools.javac.comp.Modules.allModules(Modules.java:1225)
        at jdk.javadoc/jdk.javadoc.internal.tool.ElementsTable.findModuleOfPackageName(ElementsTable.java:873)
        at jdk.javadoc/jdk.javadoc.internal.tool.ElementsTable.addPackagesFromLocations(ElementsTable.java:553)
        at jdk.javadoc/jdk.javadoc.internal.tool.ElementsTable.computeSubpackages(ElementsTable.java:526)
        at jdk.javadoc/jdk.javadoc.internal.tool.ElementsTable.computeSpecifiedPackages(ElementsTable.java:763)
        at jdk.javadoc/jdk.javadoc.internal.tool.ElementsTable.analyze(ElementsTable.java:344)
        at jdk.javadoc/jdk.javadoc.internal.tool.JavadocTool.getEnvironment(JavadocTool.java:215)
        at jdk.javadoc/jdk.javadoc.internal.tool.Start.parseAndExecute(Start.java:579)
        at jdk.javadoc/jdk.javadoc.internal.tool.Start.begin(Start.java:400)
        at jdk.javadoc/jdk.javadoc.internal.tool.Start.begin(Start.java:349)
        at jdk.javadoc/jdk.javadoc.internal.tool.Main.execute(Main.java:57)
        at jdk.javadoc/jdk.javadoc.internal.tool.Main.main(Main.java:46)
2 errors

---------- BEGIN SOURCE ----------
test.pkg.Test.java:
package test.pkg;
public class Test {
  // it does not really matter how the class looks like, as long as it is a valid java class
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
avoid the usage of -subpackages and list the package and all its subpackages instead when invoking javadoc

FREQUENCY : always