add processor to backport const keyword for open api < 3.1

Author: cdaguerre

src/Generator.php

@@ -191,6 +191,7 @@ public function getProcessors(): array
                 new Processors\MergeXmlContent(),
                 new Processors\OperationId(),
                 new Processors\CleanUnmerged(),
+                new Processors\BackportConstants(),
             ];
         }
 

src/Processors/BackportConstants.php

@@ -0,0 +1,37 @@
+<?php declare(strict_types=1);
+
+/**
+ * @license Apache 2.0
+ */
+
+namespace OpenApi\Processors;
+
+use OpenApi\Analysis;
+use OpenApi\Annotations\OpenApi;
+use OpenApi\Annotations\Property as AnnotationSchema;
+use OpenApi\Attributes\Property as AttributeSchema;
+use OpenApi\Generator;
+
+/**
+ * Transform const keywords to enum in OpenApi versions < 3.1.0.
+ */
+class BackportConstants
+{
+    public function __invoke(Analysis $analysis)
+    {
+        /** @var AnnotationSchema[] $schemas */
+        $schemas = $analysis->getAnnotationsOfType([AnnotationSchema::class, AttributeSchema::class], true);
+
+        foreach ($schemas as $schema) {
+            if (Generator::isDefault($schema->const)) {
+                continue;
+            }
+
+            if (version_compare($analysis->context->version, OpenApi::VERSION_3_1_0, '<')) {
+                $schema->type = 'enum';
+                $schema->enum = [$schema->const];
+                $schema->const = Generator::UNDEFINED;
+            }
+        }
+    }
+}