Javac inferred type does not conform to equality constraint

A DESCRIPTION OF THE PROBLEM :
In Java 11 when i try to compile code sometimes it compiles, sometimes it fails.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Compile couple times

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Success compilation
ACTUAL -
Error:(10, 58) java: incompatible types: inferred type does not conform to equality constraint(s)
    inferred: java.util.List<Main.A<?,?>>
    equality constraints(s): java.util.List<capture#1 of ? extends Main.A<?,?>>

---------- BEGIN SOURCE ----------
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Main {

    public static void main(String[] args) {
        List<? extends A<?, ?>> a = new LinkedList<>();
        Map<String, List<A<?, ?>>> b = a.stream().collect(
                Collectors.groupingBy(A::getval, Collectors.toList())
        );

        System.out.println("Hello World!");
    }

    class A<K, V> {
        String getval() {
            return "sss";
        }
    }
}
---------- END SOURCE ----------

FREQUENCY : often