[BUGFIX] Ignore f:then/f:else outside condition context

Before this change, both `f:then` and `f:else` could be used outside of
a condition tag and would then always output the content, regardless
of any condition. Now, nothing is outputted.

In the future, we might consider throwing an exception here to alert the
user about invalid Fluid syntax. However, this would be a breaking change.

Resolves: #1020

Author: s2b

src/Core/ViewHelper/AbstractConditionViewHelper.php

@@ -118,7 +118,7 @@ protected function renderThenChild()
         foreach ($this->viewHelperNode->getChildNodes() as $childNode) {
             if ($childNode instanceof ViewHelperNode
                 && str_ends_with($childNode->getViewHelperClassName(), 'ThenViewHelper')) {
-                $data = $childNode->evaluate($this->renderingContext);
+                $data = $childNode->evaluateChildNodes($this->renderingContext);
                 return $data;
             }
             if ($childNode instanceof ViewHelperNode
@@ -184,7 +184,7 @@ protected function renderElseChild()
                 $arguments = $childNode->getArguments();
                 if (isset($arguments['if'])) {
                     if ($arguments['if']->evaluate($this->renderingContext)) {
-                        return $childNode->evaluate($this->renderingContext);
+                        return $childNode->evaluateChildNodes($this->renderingContext);
                     }
                 } elseif ($elseNode === null) {
                     $elseNode = $childNode;
@@ -205,7 +205,7 @@ protected function renderElseChild()
 
         // If a f:else node exists, evaluate its content
         if ($elseNode instanceof ViewHelperNode) {
-            return $elseNode->evaluate($this->renderingContext);
+            return $elseNode->evaluateChildNodes($this->renderingContext);
         }
 
         // If only the condition is specified, but no then/else handling, the whole ViewHelper should

src/ViewHelpers/ElseViewHelper.php

@@ -50,12 +50,14 @@ public function initializeArguments()
     }
 
     /**
-     * @return string the rendered string
+     * Does nothing unless used in context of condition ViewHelper, such as f:if
+     *
      * @api
+     * @todo consider throwing an exception here in future versions
      */
-    public function render()
+    public function render(): string
     {
-        return $this->renderChildren();
+        return '';
     }
 
     /**
@@ -66,7 +68,7 @@ public function render()
      * @param TemplateCompiler $compiler
      * @return string|null
      */
-    public function compile($argumentsName, $closureName, &$initializationPhpCode, ViewHelperNode $node, TemplateCompiler $compiler)
+    public function compile($argumentsName, $closureName, &$initializationPhpCode, ViewHelperNode $node, TemplateCompiler $compiler): string
     {
         return '\'\'';
     }

src/ViewHelpers/ThenViewHelper.php

@@ -25,14 +25,14 @@ class ThenViewHelper extends AbstractViewHelper
     protected $escapeOutput = false;
 
     /**
-     * Just render everything.
+     * Does nothing unless used in context of condition ViewHelper, such as f:if
      *
-     * @return string the rendered string
      * @api
+     * @todo consider throwing an exception here in future versions
      */
-    public function render()
+    public function render(): string
     {
-        return $this->renderChildren();
+        return '';
     }
 
     /**
@@ -43,7 +43,7 @@ public function render()
      * @param TemplateCompiler $compiler
      * @return string
      */
-    public function compile($argumentsName, $closureName, &$initializationPhpCode, ViewHelperNode $node, TemplateCompiler $compiler)
+    public function compile($argumentsName, $closureName, &$initializationPhpCode, ViewHelperNode $node, TemplateCompiler $compiler): string
     {
         return '\'\'';
     }

tests/Functional/ViewHelpers/IfThenElseViewHelperTest.php

@@ -566,6 +566,27 @@ public static function renderDataProvider(): \Generator
             true,
         ];
 
+        yield 'f:then without f:if context' => [
+            '<f:then>foo</f:then>',
+            [],
+            '',
+        ];
+        yield 'f:else without f:if context' => [
+            '<f:else>foo</f:else>',
+            [],
+            '',
+        ];
+        yield 'f:else-if without f:if context, verdict true' => [
+            '<f:else if="{verdict}">foo</f:else>',
+            ['verdict' => true],
+            '',
+        ];
+        yield 'f:else-if without f:if context, verdict false' => [
+            '<f:else if="{verdict}">foo</f:else>',
+            ['verdict' => false],
+            '',
+        ];
+
         /*
          * @todo This should work but doesn't at the moment. This is probably related to the boolean
          *       parser not converting variable nodes correctly. There is a related todo in the BooleanParserTest.