Fix undefined-array-key warning in visit() for custom Node kinds

src/Language/Visitor.php

@@ -319,7 +319,9 @@ public static function visit(object $root, array $visitor, ?array $keyMap = null
                 ];
                 $inList = $node instanceof NodeList;
 
-                $keys = ($inList ? $node : $visitorKeys[$node->kind]) ?? [];
+                // Nodes with a kind not present in $visitorKeys (e.g. a custom
+                // Node subclass) are treated as leaves, i.e. no children.
+                $keys = $inList ? $node : ($visitorKeys[$node->kind] ?? []);
                 $index = -1;
                 $edits = [];
                 if ($parent !== null) {

tests/Language/VisitorTest.php

@@ -1905,4 +1905,70 @@ public function testThrowsExceptionWhenAddingIntegerToNodeList(): void
             ]
         );
     }
+
+    /**
+     * visit() must correctly invoke generic enter/leave visitors for a Node
+     * whose kind is not one of the built-in NodeKind constants (e.g. a
+     * custom Node subclass defined by a consumer).
+     */
+    public function testVisitCallsGenericVisitorsForCustomKind(): void
+    {
+        $customNode = new class([]) extends Node {
+            public string $kind = 'CustomKind';
+        };
+
+        $visited = [];
+
+        Visitor::visit(
+            $customNode,
+            [
+                'enter' => static function (Node $node) use (&$visited): void {
+                    $visited[] = ['enter', $node->kind];
+                },
+                'leave' => static function (Node $node) use (&$visited): void {
+                    $visited[] = ['leave', $node->kind];
+                },
+            ]
+        );
+
+        self::assertSame(
+            [
+                ['enter', 'CustomKind'],
+                ['leave', 'CustomKind'],
+            ],
+            $visited
+        );
+    }
+
+    /** Same as testVisitCallsGenericVisitorsForCustomKind, but for a visitor keyed specifically by the custom kind. */
+    public function testVisitCallsKindSpecificVisitorForCustomKind(): void
+    {
+        $customNode = new class([]) extends Node {
+            public string $kind = 'CustomKind';
+        };
+
+        $visited = [];
+
+        Visitor::visit(
+            $customNode,
+            [
+                'CustomKind' => [
+                    'enter' => static function (Node $node) use (&$visited): void {
+                        $visited[] = ['enter', $node->kind];
+                    },
+                    'leave' => static function (Node $node) use (&$visited): void {
+                        $visited[] = ['leave', $node->kind];
+                    },
+                ],
+            ]
+        );
+
+        self::assertSame(
+            [
+                ['enter', 'CustomKind'],
+                ['leave', 'CustomKind'],
+            ],
+            $visited
+        );
+    }
 }