deprecated "if" conditions on "for" tags

CHANGELOG

@@ -1,5 +1,6 @@
 * 2.10.0 (2019-XX-XX)
 
+ * deprecated "if" conditions on "for" tags
  * added "filter", "map", and "reduce" filters (and support for arrow functions)
  * fixed partial output leak when a PHP fatal error occurs
  * optimized context access on PHP 7.4

doc/deprecated.rst

@@ -39,6 +39,10 @@ Tags
 * The ``filter`` tag is deprecated in Twig 2.9. Use the ``apply`` tag instead
   (the ``Twig\TokenParser\FilterTokenParser`` classes is also deprecated).
 
+* Adding an ``if`` condition on a ``for`` tag is deprecated in Twig 2.10. Use a
+  ``filter`` filter or an "if" condition inside the "for" body instead (if your condition
+  depends on a variable updated inside the loop)
+
 Final Classes
 -------------
 

src/TokenParser/ForTokenParser.php

@@ -43,6 +43,8 @@ final class ForTokenParser extends AbstractTokenParser
 
         $ifexpr = null;
         if ($stream->nextIf(/* Token::NAME_TYPE */ 5, 'if')) {
+            @trigger_error(sprintf('Using an "if" condition on "for" tag is deprecated since Twig 2.10.0, use a "filter" filter or an "if" condition inside the "for" body instead (if your condition depends on a variable updated inside the loop).', __CLASS__), E_USER_DEPRECATED);
+
             $ifexpr = $this->parser->getExpressionParser()->parseExpression();
         }
 

test/Twig/Tests/Fixtures/tags/for/condition.legacy.test

@@ -1,5 +1,7 @@
 --TEST--
 "for" tag takes a condition
+--DEPRECATION--
+Using an "if" condition on "for" tag is deprecated since Twig 2.10.0, use a "filter" filter or an "if" condition inside the "for" body instead (if your condition depends on a variable updated inside the loop).
 --TEMPLATE--
 {% for i in 1..5 if i is odd -%}
     {{ loop.index }}.{{ i }}{{ foo.bar }}