Consider code like:
---
public class InvalidAnonymous {
    private void t() {
        new Undefined() {};
    }
}
---

And compile with -Xlint:overrides:
$ javac -XDdev -Xlint:overrides InvalidAnonymous.java
InvalidAnonymous.java:3: error: cannot find symbol
        new Undefined() {};
            ^
  symbol: class Undefined
  location: class InvalidAnonymous
1 error
An exception has occurred in the compiler (15-internal). Please file a bug against the Java compiler via the Java bug reporting page (http://bugreport.java.com) after checking the Bug Database (http://bugs.java.com) for duplicates. Include your program, the following diagnostic, and the parameters passed to the Java compiler in your report. Thank you.
java.lang.NullPointerException
        at jdk.compiler/com.sun.tools.javac.comp.Check.checkClassOverrideEqualsAndHash(Check.java:2148)
        at jdk.compiler/com.sun.tools.javac.comp.Check.checkClassOverrideEqualsAndHashIfNeeded(Check.java:2138)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribClass(Attr.java:5048)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribClass(Attr.java:4974)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.visitClassDef(Attr.java:965)
        at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:786)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribTree(Attr.java:663)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribStat(Attr.java:737)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.visitAnonymousClassDefinition(Attr.java:2774)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.visitNewClass(Attr.java:2665)
        at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCNewClass.accept(JCTree.java:1791)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribTree(Attr.java:663)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribExpr(Attr.java:716)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.visitExec(Attr.java:2092)
        at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCExpressionStatement.accept(JCTree.java:1523)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribTree(Attr.java:663)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribStat(Attr.java:737)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribStats(Attr.java:755)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.visitBlock(Attr.java:1414)
        at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:1038)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribTree(Attr.java:663)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribStat(Attr.java:737)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.visitMethodDef(Attr.java:1204)
        at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:884)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribTree(Attr.java:663)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribStat(Attr.java:737)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribClassBody(Attr.java:5154)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribClass(Attr.java:5045)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attribClass(Attr.java:4974)
        at jdk.compiler/com.sun.tools.javac.comp.Attr.attrib(Attr.java:4919)
        at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.attribute(JavaCompiler.java:1348)
        at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:973)
        at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:316)
        at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:176)
        at jdk.compiler/com.sun.tools.javac.Main.compile(Main.java:57)
        at jdk.compiler/com.sun.tools.javac.Main.main(Main.java:43)
printing javac parameters to: /home/jlahoda/src/jdk/jdk/javac.20200415_092643.args