javac is failing to compile this code:

abstract class Base<T> {}
abstract class Derived1<T> extends Base<T> {}
abstract class Derived2<T> extends Base<T> {
    Derived2(Derived1<T> obj) {}
}

class Test {
    Base<String> obj = new Derived2<>(new Derived1<>() { /* not abstract */ }) { /* not abstract */ };
}

issuing error message:

Test.java:8: error: Derived1 is abstract; cannot be instantiated
    Base<String> obj = new Derived2<>(new Derived1<>() { /* not abstract */ }) { /* not abstract */ };

basically during speculative attribution the class body of the anonymous classes above is removed to avoid side-effects during the process. It seems like we are checking this condition on a speculative version of the anonymous class

reported in compiler-dev: https://mail.openjdk.org/pipermail/compiler-dev/2025-June/030916.html