When implementing Java methods in Nashorn, optimistic types are not aware of the return type of the implemented method. This results in an incompatible method handle conversion from primitive to object return type:

jjs> l = new java.util.List() { iterator: function() { return foo(); }};
jdk.nashorn.javaadapters.java_util_List@7e9131d5
jjs> l.iterator()
java.lang.invoke.WrongMethodTypeException: cannot convert MethodHandle(ScriptFunction,Object)int to (ScriptFunction,Object)Iterator
java.lang.invoke.WrongMethodTypeException: cannot convert MethodHandle(ScriptFunction,Object)int to (ScriptFunction,Object)Iterator
at java.base/java.lang.invoke.MethodHandle.asTypeUncached(MethodHandle.java:858)
at java.base/java.lang.invoke.MethodHandle.asType(MethodHandle.java:844)
at jdk.dynalink/jdk.dynalink.TypeConverterFactory.asType(TypeConverterFactory.java:280)
at jdk.dynalink/jdk.dynalink.LinkerServicesImpl.asType(LinkerServicesImpl.java:131)
at jdk.scripting.nashorn/jdk.nashorn.internal.runtime.CompiledFunction.changeReturnType(CompiledFunction.java:734)
at jdk.scripting.nashorn/jdk.nashorn.internal.runtime.CompiledFunction.createInvoker(CompiledFunction.java:718)
at jdk.scripting.nashorn/jdk.nashorn.internal.runtime.CompiledFunction.access$200(CompiledFunction.java:62)
at jdk.scripting.nashorn/jdk.nashorn.internal.runtime.CompiledFunction$2.get(CompiledFunction.java:680)
at jdk.scripting.nashorn/jdk.nashorn.internal.runtime.CompiledFunction$2.get(CompiledFunction.java:677)
at jdk.scripting.nashorn/jdk.nashorn.internal.runtime.CompiledFunction.getValidOptimisticInvocation(CompiledFunction.java:623)
at jdk.scripting.nashorn/jdk.nashorn.internal.runtime.CompiledFunction.createFunctionInvocation(CompiledFunction.java:677)
at jdk.scripting.nashorn/jdk.nashorn.internal.runtime.ScriptFunction.findCallMethod(ScriptFunction.java:949)
at jdk.scripting.nashorn/jdk.nashorn.internal.runtime.ScriptObject.lookup(ScriptObject.java:1874)
at jdk.scripting.nashorn/jdk.nashorn.internal.runtime.linker.NashornLinker.getGuardedInvocation(NashornLinker.java:104)
at jdk.scripting.nashorn/jdk.nashorn.internal.runtime.linker.NashornLinker.getGuardedInvocation(NashornLinker.java:96)
at jdk.dynalink/jdk.dynalink.linker.support.CompositeTypeBasedGuardingDynamicLinker.getGuardedInvocation(CompositeTypeBasedGuardingDynamicLinker.java:184)
at jdk.dynalink/jdk.dynalink.linker.support.CompositeGuardingDynamicLinker.getGuardedInvocation(CompositeGuardingDynamicLinker.java:132)
at jdk.dynalink/jdk.dynalink.LinkerServicesImpl.lambda$getGuardedInvocation$0(LinkerServicesImpl.java:160)
at jdk.dynalink/jdk.dynalink.LinkerServicesImpl.getWithLookupInternal(LinkerServicesImpl.java:191)
at jdk.dynalink/jdk.dynalink.LinkerServicesImpl.getGuardedInvocation(LinkerServicesImpl.java:158)
at jdk.dynalink/jdk.dynalink.DynamicLinker.relink(DynamicLinker.java:265)
at jdk.nashorn.javaadapters.java_util_List.iterator(Unknown Source)
at jdk.scripting.nashorn.scripts/jdk.nashorn.internal.scripts.Script$Recompilation$3$\^shell\_/1604353554.:program(<shell>:1)
at jdk.scripting.nashorn/jdk.nashorn.internal.runtime.ScriptFunctionData.invoke(ScriptFunctionData.java:655)
at jdk.scripting.nashorn/jdk.nashorn.internal.runtime.ScriptFunction.invoke(ScriptFunction.java:513)
at jdk.scripting.nashorn/jdk.nashorn.internal.runtime.ScriptRuntime.apply(ScriptRuntime.java:517)
at jdk.scripting.nashorn/jdk.nashorn.internal.runtime.Context.eval(Context.java:861)
at jdk.scripting.nashorn/jdk.nashorn.internal.runtime.Context.eval(Context.java:796)
at jdk.scripting.nashorn.shell/jdk.nashorn.tools.jjs.Main.readEvalPrint(Main.java:218)
at jdk.scripting.nashorn/jdk.nashorn.tools.Shell.run(Shell.java:175)
at jdk.scripting.nashorn.shell/jdk.nashorn.tools.jjs.Main.main(Main.java:104)
at jdk.scripting.nashorn.shell/jdk.nashorn.tools.jjs.Main.main(Main.java:80)