Wrong "incompatible generic type" error

package test;

import java.io.Serializable;

public final class Pair<L, R> implements Serializable {

	public static <L, R> Pair<L, R> of(final L left, final R right) {
		return new Pair<>(left, right);
	}

	public final L left;
	public final R right;

	private Pair(final L left, final R right) {
		this.left = left;
		this.right = right;
	}
}

package test2

import test.Pair

import java.util.Date;

import groovy.transform.CompileStatic

@CompileStatic
class Test {

	Pair<Pair<Integer, Pair<String, Date>>, Pair<Integer, Pair<String, Date>>> doSmething() {
		def left = (Pair<Integer, Pair<String, Date>>) null
		def right = (Pair<Integer, Pair<String, Date>>) null
		return Pair.of(left, right)
	}
}