Add test case ensuring we don't incur in FNs

pmd-java/src/test/resources/net/sourceforge/pmd/lang/java/rule/bestpractices/xml/UnusedPrivateMethod.xml

@@ -2297,4 +2297,20 @@ public class ObtainViaTest {
             ]]></code>
     </test-code>
 
+    <test-code>
+        <description>UnusedPrivateMethod #5083 - method reference without target type (3)</description>
+        <expected-problems>2</expected-problems>
+        <code><![CDATA[
+import java.util.Set;
+
+public class Foo {
+    private void foo(AnotherUnresolvedType baz) {}
+    private void foo(UnresolvedType baz) {}
+    public void bar(Set<UnresolvedType> s) {
+        s.forEach(this::foo);  // Not being able to resolve UnresolvedType and AnotherUnresolvedType means we can't tell which foo is being used here…
+    }
+}
+            ]]></code>
+    </test-code>
+
 </test-data>