fix DualIterMap context value handling

Author: GuntherRademacher

basex-core/src/main/java/org/basex/query/expr/DualIterMap.java

@@ -71,6 +71,7 @@ public Value value(final QueryContext qc) throws QueryException {
         qf.value = item1;
         final Iter iter2 = exprs[1].iter(qc);
         for(Item item2; (item2 = qc.next(iter2)) != null;) vb.add(item2);
+        qf.value = qv;
       }
       return vb.value(this);
     } finally {

basex-core/src/test/java/org/basex/query/ast/RewritingsTest.java

@@ -3398,4 +3398,19 @@ private static String gh1852(final String query) {
     query("let $a := 'x' let $b := if($a) then (<a/>, <b/>) else error() return $b/name()",
         "a\nb");
   }
+
+  /** DualIterMap context value setting. */
+  @Test public void gh2569() {
+    check("let $node := <X><A/><B/><C/></X>\n"
+        + "let $pos := $node/A ! position()\n"
+        + "let $nodes := for $c at $i in $node/* where $i gt $pos return $c\n"
+        + "let $flat := $nodes ! (if (*) then * else .)\n"
+        + "return element Y {$flat except $flat[1]}",
+        "<Y><C/></Y>", exists(DualIterMap.class));
+    check("let $node := <X><A/><B/><C/></X>\n"
+        + "return $node/A ! position()\n"
+        + "  -> (for $c at $i in $node/* where $i gt . return $c) ! (* otherwise .)\n"
+        + "  -> element Y {. except head(.)}",
+        "<Y><C/></Y>", exists(DualIterMap.class));
+  }
 }