JRuby 9.3 singleton_class bug

[JacobEvelyn]

Hi! I believe I've found a bug that's resulting in JRuby tests failing for panorama-ed/memo_wise that were passing with JRuby 9.2 (and which also pass in all MRI versions).

Environment Information

Provide at least:

• JRuby version (jruby -v): jruby 9.3.1.0
• Operating system and platform (e.g. uname -a): Ubuntu 20.04.3 LTS (but have also reproduced on macOS 11.6

Expected Behavior

ObjectSpace.each_object(Module).each(&:singleton_class)

should not crash the Ruby interpreter

Actual Behavior

Unhandled Java exception: java.lang.UnsupportedOperationException: An included class is only a wrapper for a module
java.lang.UnsupportedOperationException: An included class is only a wrapper for a module
            setMetaClass at org/jruby/PrependedModule.java:119
           makeMetaClass at org/jruby/RubyBasicObject.java:548
        toSingletonClass at org/jruby/RubyClass.java:475
        toSingletonClass at org/jruby/MetaClass.java:112
       getSingletonClass at org/jruby/RubyBasicObject.java:532
         singleton_class at org/jruby/RubyKernel.java:1981
                    call at org/jruby/RubyKernel$INVOKER$s$0$0$singleton_class.gen:-1
            cacheAndCall at org/jruby/runtime/callsite/CachingCallSite.java:351
                    call at org/jruby/runtime/callsite/CachingCallSite.java:144
           yieldSpecific at org/jruby/RubySymbol.java:1467
                 doYield at org/jruby/RubySymbol.java:1460
                   yield at org/jruby/runtime/BlockBody.java:116
             yieldValues at org/jruby/runtime/Block.java:234
                    call at org/jruby/RubyEnumerator.java:391
                    call at org/jruby/runtime/BlockCallback.java:58
                 doYield at org/jruby/runtime/CallBlock.java:91
                   yield at org/jruby/runtime/BlockBody.java:108
                   yield at org/jruby/runtime/Block.java:188
     each_objectInternal at org/jruby/RubyObjectSpace.java:157
             each_object at org/jruby/RubyObjectSpace.java:189
                    call at org/jruby/RubyObjectSpace$INVOKER$s$0$1$each_object.gen:-1
  finvokeWithRefinements at org/jruby/RubyClass.java:510
                 finvoke at org/jruby/RubyClass.java:498
                  invoke at org/jruby/runtime/Helpers.java:645
              callMethod at org/jruby/RubyBasicObject.java:387
                __each__ at org/jruby/RubyEnumerator.java:387
                    each at org/jruby/RubyEnumerator.java:383
                    call at org/jruby/RubyEnumerator$INVOKER$i$each.gen:-1
            cacheAndCall at org/jruby/runtime/callsite/CachingCallSite.java:362
                    call at org/jruby/runtime/callsite/CachingCallSite.java:154
       invokeOther1:each at path/to/jruby-test.rb:1
             RUBY$script at path/to/jruby-test.rb:1
                     run at path/to/jruby-test.rb:-1
     invokeWithArguments at java/lang/invoke/MethodHandle.java:627
                    load at org/jruby/ir/Compiler.java:114
               runScript at org/jruby/Ruby.java:1257
             runNormally at org/jruby/Ruby.java:1176
             runNormally at org/jruby/Ruby.java:1158
             runNormally at org/jruby/Ruby.java:1194
             runFromMain at org/jruby/Ruby.java:977
           doRunFromMain at org/jruby/Main.java:405
             internalRun at org/jruby/Main.java:289
                     run at org/jruby/Main.java:234
                    main at org/jruby/Main.java:206

[headius]

I believe this indicates that we are walking IncludedModuleWrapper instances when walking all modules during each_object. Investigating.

[headius]

I pushed #6900 to fix this by avoiding prepended module wrappers in the same way we avoid included module wrappers.

I did not see any specs for this particular behavior, but I'm also not sure exactly what to test here.