gistfile1.txt

java.lang.ClassCastException: com.sun.tools.javac.code.Type$AnnotatedType cannot be cast to com.sun.tools.javac.code.Type$ClassType
        at com.sun.tools.javac.api.JavacTrees$3.visitErrorType(JavacTrees.java:689)
        at com.sun.tools.javac.api.JavacTrees$3.visitErrorType(JavacTrees.java:646)
        at com.sun.tools.javac.code.Type$ErrorType.accept(Type.java:1829)
        at com.sun.tools.javac.code.Types$DefaultTypeVisitor.visit(Types.java:4571)
        at com.sun.tools.javac.api.JavacTrees.fuzzyMatch(JavacTrees.java:642)
        at com.sun.tools.javac.api.JavacTrees.fuzzyMatch(JavacTrees.java:633)
        at com.sun.tools.javac.api.JavacTrees.hasParameterTypes(JavacTrees.java:625)
        at com.sun.tools.javac.api.JavacTrees.searchMethod(JavacTrees.java:572)
        at com.sun.tools.javac.api.JavacTrees.findMethod(JavacTrees.java:528)
        at com.sun.tools.javac.api.JavacTrees.attributeDocReference(JavacTrees.java:414)
        at com.sun.tools.javac.api.JavacTrees.getElement(JavacTrees.java:345)
        at com.sun.tools.doclint.Checker.visitReference(Checker.java:773)
        at com.sun.tools.doclint.Checker.visitReference(Checker.java:95)
        at com.sun.tools.javac.tree.DCTree$DCReference.accept(DCTree.java:523)
        at com.sun.source.util.DocTreePathScanner.scan(DocTreePathScanner.java:65)
        at com.sun.source.util.DocTreeScanner.visitLink(DocTreeScanner.java:177)
        at com.sun.tools.doclint.Checker.visitLink(Checker.java:701)
        at com.sun.tools.doclint.Checker.visitLink(Checker.java:95)
        at com.sun.tools.javac.tree.DCTree$DCLink.accept(DCTree.java:422)
        at com.sun.source.util.DocTreePathScanner.scan(DocTreePathScanner.java:65)
        at com.sun.source.util.DocTreeScanner.scanAndReduce(DocTreeScanner.java:81)
        at com.sun.source.util.DocTreeScanner.scan(DocTreeScanner.java:92)
        at com.sun.source.util.DocTreeScanner.scanAndReduce(DocTreeScanner.java:100)
        at com.sun.source.util.DocTreeScanner.visitDocComment(DocTreeScanner.java:140)
        at com.sun.tools.doclint.Checker.visitDocComment(Checker.java:217)
        at com.sun.tools.doclint.Checker.visitDocComment(Checker.java:95)
        at com.sun.tools.javac.tree.DCTree$DCDocComment.accept(DCTree.java:122)
        at com.sun.source.util.DocTreePathScanner.scan(DocTreePathScanner.java:47)
        at com.sun.tools.doclint.Checker.scan(Checker.java:179)
        at com.sun.tools.doclint.DocLint.scan(DocLint.java:319)
        at com.sun.tools.javadoc.DocImpl.comment(DocImpl.java:143)
        at com.sun.tools.javadoc.DocImpl.tags(DocImpl.java:164)
        at com.sun.tools.doclets.internal.toolkit.util.VisibleMemberMap$ClassMembers.checkOnPropertiesTags(VisibleMemberMap.java:691)
        at com.sun.tools.doclets.internal.toolkit.util.VisibleMemberMap$ClassMembers.getClassMembers(VisibleMemberMap.java:464)
        at com.sun.tools.doclets.internal.toolkit.util.VisibleMemberMap$ClassMembers.addMembers(VisibleMemberMap.java:372)
        at com.sun.tools.doclets.internal.toolkit.util.VisibleMemberMap$ClassMembers.mapClass(VisibleMemberMap.java:346)
        at com.sun.tools.doclets.internal.toolkit.util.VisibleMemberMap$ClassMembers.build(VisibleMemberMap.java:341)
        at com.sun.tools.doclets.internal.toolkit.util.VisibleMemberMap$ClassMembers.access$100(VisibleMemberMap.java:291)
        at com.sun.tools.doclets.internal.toolkit.util.VisibleMemberMap.<init>(VisibleMemberMap.java:127)
20 errors
23 warnings
        at com.sun.tools.doclets.internal.toolkit.builders.MemberSummaryBuilder.<init>(MemberSummaryBuilder.java:82)
        at com.sun.tools.doclets.internal.toolkit.builders.MemberSummaryBuilder.getInstance(MemberSummaryBuilder.java:101)
        at com.sun.tools.doclets.internal.toolkit.builders.BuilderFactory.getMemberSummaryBuilder(BuilderFactory.java:278)
        at com.sun.tools.doclets.formats.html.ClassWriterImpl.getNavSummaryLinks(ClassWriterImpl.java:641)
        at com.sun.tools.doclets.formats.html.ClassWriterImpl.addSummaryDetailLinks(ClassWriterImpl.java:622)
        at com.sun.tools.doclets.formats.html.HtmlDocletWriter.addNavLinks(HtmlDocletWriter.java:560)
        at com.sun.tools.doclets.formats.html.ClassWriterImpl.getHeader(ClassWriterImpl.java:165)
        at com.sun.tools.doclets.internal.toolkit.builders.ClassBuilder.buildClassDoc(ClassBuilder.java:145)
        at sun.reflect.GeneratedMethodAccessor5.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at com.sun.tools.doclets.internal.toolkit.builders.AbstractBuilder.invokeMethod(AbstractBuilder.java:187)
        at com.sun.tools.doclets.internal.toolkit.builders.AbstractBuilder.build(AbstractBuilder.java:135)
        at com.sun.tools.doclets.internal.toolkit.builders.ClassBuilder.build(ClassBuilder.java:120)
        at com.sun.tools.doclets.formats.html.HtmlDoclet.generateClassFiles(HtmlDoclet.java:189)
        at com.sun.tools.doclets.internal.toolkit.AbstractDoclet.generateClassFiles(AbstractDoclet.java:207)
        at com.sun.tools.doclets.internal.toolkit.AbstractDoclet.generateClassFiles(AbstractDoclet.java:191)
        at com.sun.tools.doclets.internal.toolkit.AbstractDoclet.startGeneration(AbstractDoclet.java:139)
        at com.sun.tools.doclets.internal.toolkit.AbstractDoclet.start(AbstractDoclet.java:82)
        at com.sun.tools.doclets.formats.html.HtmlDoclet.start(HtmlDoclet.java:80)
        at com.sun.tools.doclets.standard.Standard.start(Standard.java:39)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at com.sun.tools.javadoc.DocletInvoker.invoke(DocletInvoker.java:310)
        at com.sun.tools.javadoc.DocletInvoker.start(DocletInvoker.java:189)
        at com.sun.tools.javadoc.Start.parseAndExecute(Start.java:366)
        at com.sun.tools.javadoc.Start.begin(Start.java:219)
        at com.sun.tools.javadoc.Start.begin(Start.java:205)
        at com.sun.tools.javadoc.Main.execute(Main.java:64)
        at com.sun.tools.javadoc.Main.main(Main.java:54)
javadoc: error - com.sun.tools.javac.code.Type$AnnotatedType cannot be cast to com.sun.tools.javac.code.Type$ClassType

To any time travellers out there: I fixed this by using Java 17 for the javadoc gradle task. So:

tasks.withType(Javadoc) {
    javadocTool = javaToolchains.javadocToolFor {
        languageVersion = JavaLanguageVersion.of(17)
    }
}