phpGH-16315: Improve error messages when extending enums

Closes phpGH-16491

Author: DanielEScherzer

Zend/tests/enum/extending-builtin-error.phpt

@@ -0,0 +1,10 @@
+--TEST--
+GH-16315: Extending built-in enum
+--FILE--
+<?php
+
+class Demo extends RoundingMode {}
+
+?>
+--EXPECTF--
+Fatal error: Class Demo cannot extend enum RoundingMode in %s on line 3

Zend/tests/enum/extending-user-error.phpt

@@ -0,0 +1,12 @@
+--TEST--
+GH-16315: Extending userland enum
+--FILE--
+<?php
+
+enum MyEnum {}
+
+class Demo extends MyEnum {}
+
+?>
+--EXPECTF--
+Fatal error: Class Demo cannot extend enum MyEnum in %s on line 5

Zend/tests/enum/final.phpt

@@ -5,8 +5,9 @@ Enum is final
 
 enum Foo {}
 
-class Bar extends Foo {}
+$final = new ReflectionClass(Foo::class)->isFinal();
+var_dump($final);
 
 ?>
---EXPECTF--
-Fatal error: Class Bar cannot extend final class Foo in %s on line %d
+--EXPECT--
+bool(true)

Zend/zend_inheritance.c

@@ -1757,7 +1757,12 @@ ZEND_API void zend_do_inheritance_ex(zend_class_entry *ce, zend_class_entry *par
 		if (UNEXPECTED(!(parent_ce->ce_flags & ZEND_ACC_INTERFACE))) {
 			zend_error_noreturn(E_COMPILE_ERROR, "Interface %s cannot extend class %s", ZSTR_VAL(ce->name), ZSTR_VAL(parent_ce->name));
 		}
-	} else if (UNEXPECTED(parent_ce->ce_flags & (ZEND_ACC_INTERFACE|ZEND_ACC_TRAIT|ZEND_ACC_FINAL))) {
+	} else if (UNEXPECTED(parent_ce->ce_flags & (ZEND_ACC_INTERFACE|ZEND_ACC_TRAIT|ZEND_ACC_FINAL|ZEND_ACC_ENUM))) {
+		/* Class must not extend an enum (GH-16315); check enums first since
+		 * enums are implemented as final classes */
+		if (parent_ce->ce_flags & ZEND_ACC_ENUM) {
+			zend_error_noreturn(E_COMPILE_ERROR, "Class %s cannot extend enum %s", ZSTR_VAL(ce->name), ZSTR_VAL(parent_ce->name));
+		}
 		/* Class must not extend a final class */
 		if (parent_ce->ce_flags & ZEND_ACC_FINAL) {
 			zend_error_noreturn(E_COMPILE_ERROR, "Class %s cannot extend final class %s", ZSTR_VAL(ce->name), ZSTR_VAL(parent_ce->name));