BUGFIX check if parent context is SiteTree instance

fixes #1913

code/Model/SiteTree.php

@@ -1155,7 +1155,8 @@ class SiteTree extends DataObject implements PermissionProvider, i18nEntityProvi
         // Check parent (custom canCreate option for SiteTree)
         // Block children not allowed for this parent type
         $parent = isset($context['Parent']) ? $context['Parent'] : null;
-        if ($parent && !in_array(static::class, $parent->allowedChildren())) {
+        $strictParentInstance = ($parent && $parent instanceof SiteTree);
+        if ($strictParentInstance && !in_array(static::class, $parent->allowedChildren())) {
             return false;
         }
 
@@ -1171,7 +1172,7 @@ class SiteTree extends DataObject implements PermissionProvider, i18nEntityProvi
         }
 
         // Fall over to inherited permissions
-        if ($parent && $parent->exists()) {
+        if ($strictParentInstance && $parent->exists()) {
             return $parent->canAddChildren($member);
         } else {
             // This doesn't necessarily mean we are creating a root page, but that

tests/model/SiteTreeTest.php

@@ -50,6 +50,7 @@ class SiteTreeTest extends SapphireTest
         SiteTreeTest_ClassCext::class,
         SiteTreeTest_NotRoot::class,
         SiteTreeTest_StageStatusInherit::class,
+        SiteTreeTest_DataObject::class,
     );
 
     public function testCreateDefaultpages()
@@ -632,6 +633,9 @@ class SiteTreeTest extends SapphireTest
         // Test creation underneath a parent which doesn't exist in the database. This should
         // fall back to checking whether the user can create pages at the root of the site
         $this->assertTrue(singleton(SiteTree::class)->canCreate(null, array('Parent' => singleton(SiteTree::class))));
+
+        //Test we don't check for allowedChildren on parent context if it's not SiteTree instance
+        $this->assertTrue(singleton(SiteTree::class)->canCreate(null, ['Parent' => $this->objFromFixture('SiteTreeTest_DataObject', 'relations')]));
     }
 
     public function testEditPermissionsOnDraftVsLive()

tests/model/SiteTreeTest.yml

@@ -123,3 +123,8 @@ SilverStripe\CMS\Model\RedirectorPage:
     URLSegment: external
     RedirectionType: External
     ExternalURL: "http://www.google.com?a&b"
+
+SiteTreeTest_DataObject:
+  relations:
+    Title: 'Linked DataObject'
+    Pages: =>Page.home,=>Page.about,=>Page.staff

tests/model/SiteTreeTest_DataObject.php

@@ -0,0 +1,28 @@
+<?php
+
+use SilverStripe\CMS\Model\SiteTree;
+use SilverStripe\Dev\TestOnly;
+use SilverStripe\ORM\DataObject;
+
+/**
+ * Class SiteTreeTest_DataObject
+ *
+ * @property string $Title
+ * @method Pages[]|ManyManyList $Pages
+ */
+class SiteTreeTest_DataObject extends DataObject implements TestOnly
+{
+    /**
+     * @var array
+     */
+    private static $db = [
+        'Title' => 'Varchar',
+    ];
+
+    /**
+     * @var array
+     */
+    private static $many_many = [
+        'Pages' => SiteTree::class,
+    ];
+}