OK, this can be checked once

src/Tokenizer/Tokens.php

@@ -858,6 +858,13 @@ public function insertSlices(array $slices): void
         $this->setSize($oldSize + $itemsCount);
 
         krsort($slices);
+        $farthestSliceIndex = key($slices);
+
+        // We check only the farthest index, if it's within the size of collection, other indices will be valid too.
+        if (!\is_int($farthestSliceIndex) || $farthestSliceIndex > $oldSize) {
+            throw new \OutOfBoundsException(sprintf('Cannot insert index "%s" outside of collection.', $farthestSliceIndex));
+        }
+
         $previousSliceIndex = $oldSize;
 
         // since we only move already existing items around, we directly call into SplFixedArray::offset* methods.
@@ -867,10 +874,6 @@ public function insertSlices(array $slices): void
                 throw new \OutOfBoundsException(sprintf('Invalid index "%s".', $index));
             }
 
-            if ($index > $oldSize) {
-                throw new \OutOfBoundsException(sprintf('Cannot insert index "%s" outside of collection.', $index));
-            }
-
             $slice = \is_array($slice) || $slice instanceof self ? $slice : [$slice];
             $sliceCount = \count($slice);