When C14N fails try comparison without canonicalization

src/DOMNodeComparator.php

@@ -70,8 +70,15 @@ public function assertEquals($expected, $actual, $delta = 0.0, $canonicalize = f
     private function nodeToText(DOMNode $node, bool $canonicalize, bool $ignoreCase): string
     {
         if ($canonicalize) {
+            /** @psalm-var string|false $c14n */
+            $c14n = @$node->C14N();
+
+            if ($c14n === false || $c14n === '') { // try node-to-text without canonicalize
+                return $this->nodeToText($node, false, $ignoreCase);
+            }
+
             $document = new DOMDocument;
-            @$document->loadXML($node->C14N());
+            $document->loadXML($c14n);
 
             $node = $document;
         }

tests/DOMNodeComparatorTest.php

@@ -121,6 +121,50 @@ public function assertEqualsFailsProvider()
         ];
     }
 
+    public function assertEqualsSucceedsWithNonCanonicalizableNodesProvider()
+    {
+        $document = new DOMDocument;
+
+        return [
+            [$document, new DOMDocument],
+            [$document->createElement('foo'), $document->createElement('foo')],
+            [
+                $this->createDOMDocument(
+                    '<!DOCTYPE foo SYSTEM "foo.dtd" [<!ENTITY ns_foo "http://uri.tld/foo">]><i:foo xmlns:i="&ns_foo;"/>'
+                ),
+                $this->createDOMDocument(
+                    '<!DOCTYPE foo SYSTEM "foo.dtd" [<!ENTITY ns_foo "http://uri.tld/foo">]><i:foo xmlns:i="&ns_foo;"/>'
+                ),
+            ],
+        ];
+    }
+
+    public function assertEqualsFailsWithNonCanonicalizableNodesProvider()
+    {
+        // empty document makes C14N return empty string
+        $document = new DOMDocument;
+
+        // nodes created but not appended to the document makes C14N return empty string
+        $nodeFoo = $document->createElement('foo');
+        $nodeBar = $document->createElement('bar');
+
+        // documents with xmlns definitions to xml entities makes C14N return false
+        $documentNsUriIsEntityFoo = $this->createDOMDocument( // root element is i:foo
+            '<!DOCTYPE foo SYSTEM "foo.dtd" [<!ENTITY ns_foo "http://uri.tld/foo">]><i:foo xmlns:i="&ns_foo;"/>'
+        );
+        $documentNsUriIsEntityBar = $this->createDOMDocument( // root element is i:bar
+            '<!DOCTYPE foo SYSTEM "foo.dtd" [<!ENTITY ns_foo "http://uri.tld/foo">]><i:bar xmlns:i="&ns_foo;"/>'
+        );
+
+        return [
+            [$document, $nodeFoo],
+            [$document, $documentNsUriIsEntityFoo],
+            [$nodeFoo, $nodeBar],
+            [$nodeFoo, $documentNsUriIsEntityFoo],
+            [$documentNsUriIsEntityFoo, $documentNsUriIsEntityBar],
+        ];
+    }
+
     /**
      * @dataProvider acceptsSucceedsProvider
      */
@@ -169,6 +213,22 @@ public function testAssertEqualsFails($expected, $actual): void
         $this->comparator->assertEquals($expected, $actual);
     }
 
+    /**
+     * @dataProvider assertEqualsSucceedsWithNonCanonicalizableNodesProvider
+     */
+    public function testAssertEqualsSucceedsWithNonCanonicalizableNodes($expected, $actual): void
+    {
+        $this->testAssertEqualsSucceeds($expected, $actual);
+    }
+
+    /**
+     * @dataProvider assertEqualsFailsWithNonCanonicalizableNodesProvider
+     */
+    public function testAssertEqualsFailsWithNonCanonicalizableNodes($expected, $actual): void
+    {
+        $this->testAssertEqualsFails($expected, $actual);
+    }
+
     private function createDOMDocument($content)
     {
         $document                     = new DOMDocument;