Allow class inheritance determination for enums

### Motivation

In the scope of [a downstream project issue](Crell/Serde#10), It was noticed that in cases where an enum implemented an interface, the interface attributes could not be read.

This change allows `ReflectionEnum` to retrieve such information if it exists

Author: agustingomes

README.md

@@ -818,6 +818,64 @@ In this case, a class may be marked with either `Screen` or `Audio`, but not bot
 
 In this example, `$displayInfoA->type` will be an instance of `Screen`, `$displayInfoB->type` will be an instance of `Audio`, and `$displayInfoC->type` will be `null`.
 
+### Interface using attributes
+
+Interfaces may also implement attributes. This is useful for cases in which you want to define a type map at the interface level:
+
+Note: in this case, make sure the class implements the `Inheritable` interface, to return the expected values.
+
+```php
+
+use Crell\AttributeUtils\Inheritable;
+
+#[Attribute(Attribute::TARGET_CLASS)]
+final readonly class TypeMap implements Inheritable
+{
+    /**
+     * @param non-empty-string $key
+     * @param array<non-empty-string, class-string> $map
+     */
+    public function __construct(
+        public string $key,
+        public array $map,
+    ) {
+    }
+}
+
+#[TypeMap(
+    key: 'type',
+    map: [
+        'enum'   => MyEnum::class,
+        'object' => MyObject::class,
+    ]
+)]
+interface Something
+{
+}
+
+enum MyEnum: string implements Something
+{
+    case A = 'a';
+    case B = 'b';
+}
+
+class MyObject implements Something
+{
+    public function __construct(
+        public int $id,
+    ) {
+    }
+}
+
+$analyzer = new Crell\AttributeUtils\Analyzer();
+$attrib   = $analyzer->analyze(MyEnum::class, TypeMap::class);
+
+//// $attrib is now an instance of Map.
+print $attrib->key . PHP_EOL; // Prints 'type'
+print $attrib->map['enum'] . PHP_EOL; // Prints `MyEnum`
+print $attrib->map['object'] . PHP_EOL; // Prints `MyObject`
+```
+
 ## Change log
 
 Please see [CHANGELOG](CHANGELOG.md) for more information on what has changed recently.

src/AttributeParser.php

@@ -224,8 +224,7 @@ protected function attributeInheritanceTree(\Reflector $subject, string $attribu
                 \ReflectionMethod::class => $this->classElementInheritanceTree($subject),
                 \ReflectionClassConstant::class => $this->classElementInheritanceTree($subject),
                 \ReflectionParameter::class => $this->parameterInheritanceTree($subject),
-                // If it's an enum, there's nothing to inherit so just stub that out.
-                \ReflectionEnum::class => [],
+                \ReflectionEnum::class => $this->enumInheritanceTree($subject),
             };
         }
     }
@@ -248,6 +247,24 @@ protected function classInheritanceTree(\ReflectionClass $subject): iterable
         }
     }
 
+    /**
+     * Returns all the ReflectionClasses in an enum subject's inheritance tree.
+     *
+     * This includes only interfaces, as enums cannot extend other classes.
+     *
+     * @param \ReflectionEnum<\UnitEnum> $subject
+     *   The reflection of the enum for which we want the ancestors.
+     * @return iterable<\ReflectionClass<object>>
+     * @throws \ReflectionException
+     */
+    protected function enumInheritanceTree(\ReflectionEnum $subject): iterable
+    {
+        $ancestors = $this->classAncestors($subject->getName(), false);
+        foreach ($ancestors as $ancestor) {
+            yield new \ReflectionClass($ancestor);
+        }
+    }
+
     /**
      * Returns all of the ReflectionParameters in a subject's inheritance tree.
      *

tests/ClassAnalyzerTest.php

@@ -49,6 +49,7 @@
 use Crell\AttributeUtils\Records\ClassWithScopesMulti;
 use Crell\AttributeUtils\Records\ClassWithScopesNotDefault;
 use Crell\AttributeUtils\Records\ClassWithSubAttributes;
+use Crell\AttributeUtils\Records\EnumWithInterface;
 use Crell\AttributeUtils\Records\LabeledApp;
 use Crell\AttributeUtils\Records\MissingPropertyAttributeArguments;
 use Crell\AttributeUtils\Records\MultiuseClass;
@@ -416,6 +417,15 @@ public static function attributeTestProvider(): \Generator
             },
         ];
 
+        yield 'Enum implementing interface' => [
+            'subject' => EnumWithInterface::class,
+            'attribute' => BasicClass::class,
+            'test' => static function(mixed $classDef) {
+                self::assertEquals(5, $classDef->a);
+                self::assertEquals(10, $classDef->b);
+            },
+        ];
+
         yield 'Field takes defaults from class' => [
             'subject' => PropertyThatTakesClassDefault::class,
             'attribute' => PropertyTakesClassDefaultClass::class,

tests/Records/EnumWithInterface.php

@@ -0,0 +1,11 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Crell\AttributeUtils\Records;
+
+enum EnumWithInterface: int implements AnInterface
+{
+    case A = 1;
+    case B = 2;
+}