new-features.xml Add an example of cyclic dependencies (#4501)

Author: mmalferov

appendices/migration74/new-features.xml

@@ -77,6 +77,33 @@ class ChildProducer extends Producer {
     Full variance support is only available if autoloading is used. Inside a
     single file only non-cyclic type references are possible, because all
     classes need to be available before they are referenced.
+    <informalexample>
+     <programlisting role="php">
+<![CDATA[
+<?php
+
+/**
+ * These classes satisfy the LSP requirements, because C is a subtype of A.
+ * However, at the time class B is declared, class C is not yet available
+ */
+class A
+{
+    public function method(): A {}
+}
+
+class B extends A
+{
+    // Fatal error: Could not check compatibility between B::method():C and
+    // A::method(): A, because class С is not available
+    public function method(): С {}
+}
+
+class C extends B {}
+
+?>
+]]>
+     </programlisting>
+    </informalexample>
    </para>
   </sect3>