A DESCRIPTION OF THE PROBLEM :
An example of an unsound program: https://github.com/scala/scala3/issues/16789#issuecomment-1410721692.
Essentially, the bug is that Supplier<Magic<?>> (or more precisely Function<Integer,Magic<CAP#1>>) can be passed to a function that accepts Supplier<Magic<T>>.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Write code from link in description oк from test case, compile it and run program.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Compilation error
ACTUAL -
The program compiles and a ClassNotFoundError error is thrown at runtime.

---------- BEGIN SOURCE ----------
import java.util.function.Function;

public class Main {
    public static void main(String[] args) {
        onestep(num -> {
            Magic<?> m = num > 0 ? new IntMagic() : new StrMagic();
            return m;
        });
    }

    static <T> T onestep(Function<Integer, Magic<T>> m) {
        return m.apply(1).step(m.apply(0).init());
    }
}

interface Magic<S> {
    S init();
    S step(S s);
}

class IntMagic implements Magic<Integer> {
    public Integer init() { return 0; }
    public Integer step(Integer s) { return s + 1; }
}

class StrMagic implements Magic<String> {
    public String init() { return ""; }
    public String step(String s) { return s + "0"; }
}
---------- END SOURCE ----------