ObjectType: fix isEnum

src/Type/ObjectType.php

@@ -6,6 +6,7 @@
 use ArrayObject;
 use Closure;
 use Countable;
+use DateTimeInterface;
 use Iterator;
 use IteratorAggregate;
 use PHPStan\Analyser\OutOfClassScope;
@@ -44,6 +45,8 @@
 use PHPStan\Type\Traits\NonGeneralizableTypeTrait;
 use PHPStan\Type\Traits\NonGenericTypeTrait;
 use PHPStan\Type\Traits\UndecidedComparisonTypeTrait;
+use Stringable;
+use Throwable;
 use Traversable;
 use function array_key_exists;
 use function array_map;
@@ -730,7 +733,23 @@ public function isEnum(): TrinaryLogic
 			return TrinaryLogic::createMaybe();
 		}
 
-		return TrinaryLogic::createFromBoolean($classReflection->isEnum());
+		if (
+			$classReflection->isEnum()
+			|| $classReflection->is('UnitEnum')
+		) {
+			return TrinaryLogic::createYes();
+		}
+
+		if (
+			$classReflection->isInterface()
+			&& !$classReflection->is(Stringable::class) // enums cannot have __toString
+			&& !$classReflection->is(Throwable::class) // enums cannot extend Exception/Error
+			&& !$classReflection->is(DateTimeInterface::class) // userland classes cannot extend DateTimeInterface
+		) {
+			return TrinaryLogic::createMaybe();
+		}
+
+		return TrinaryLogic::createNo();
 	}
 
 	public function canAccessProperties(): TrinaryLogic

tests/PHPStan/Type/ObjectTypeTest.php

@@ -70,6 +70,35 @@ public function testIsIterable(ObjectType $type, TrinaryLogic $expectedResult):
 		);
 	}
 
+	/**
+	 * @return iterable<array{0: ObjectType, 1: TrinaryLogic}>
+	 */
+	public function dataIsEnum(): iterable
+	{
+		if (PHP_VERSION_ID >= 80000) {
+			yield [new ObjectType('UnitEnum'), TrinaryLogic::createYes()];
+			yield [new ObjectType('BackedEnum'), TrinaryLogic::createYes()];
+		}
+		yield [new ObjectType('Unknown'), TrinaryLogic::createMaybe()];
+		yield [new ObjectType('Countable'), TrinaryLogic::createMaybe()];
+		yield [new ObjectType('Stringable'), TrinaryLogic::createNo()];
+		yield [new ObjectType('Throwable'), TrinaryLogic::createNo()];
+		yield [new ObjectType('DateTime'), TrinaryLogic::createNo()];
+	}
+
+	/**
+	 * @dataProvider dataIsEnum
+	 */
+	public function testIsEnum(ObjectType $type, TrinaryLogic $expectedResult): void
+	{
+		$actualResult = $type->isEnum();
+		$this->assertSame(
+			$expectedResult->describe(),
+			$actualResult->describe(),
+			sprintf('%s -> isEnum()', $type->describe(VerbosityLevel::precise())),
+		);
+	}
+
 	public function dataIsCallable(): array
 	{
 		return [