Promote "Cannot use parent" to fatal error

Author: nikic

UPGRADING

@@ -45,6 +45,8 @@ PHP 8.0 UPGRADE NOTES
   . The default error_reporting level is now E_ALL. Previously it excluded
     E_NOTICE and E_DEPRECATED.
   . display_startup_errors is now enabled by default.
+  . Using "parent" inside a class that has no parent will now result in a
+    fatal compile-time error.
   . The @ operator will no longer silence fatal errors (E_ERROR, E_CORE_ERROR,
     E_COMPILE_ERROR, E_USER_ERROR, E_RECOVERABLE_ERROR, E_PARSE). Error handlers
     that expect error_reporting to be 0 when @ is used, should be adjusted to

Zend/tests/class_name_as_scalar_error_002.phpt

@@ -11,9 +11,4 @@ namespace Foo\Bar {
 }
 ?>
 --EXPECTF--
-Deprecated: Cannot use "parent" when current class scope has no parent in %s on line %d
-
-Fatal error: Uncaught Error: Cannot use "parent" when current class scope has no parent in %s:%d
-Stack trace:
-#0 {main}
-  thrown in %s on line %d
+Fatal error: Cannot use "parent" when current class scope has no parent in %s on line %d

Zend/tests/type_declarations/variance/parent_in_class_failure1.phpt

@@ -13,6 +13,4 @@ class B extends A {
 
 ?>
 --EXPECTF--
-Deprecated: Cannot use "parent" when current class scope has no parent in %s on line %d
-
-Fatal error: Could not check compatibility between B::method(A $x) and A::method(parent $x), because class parent is not available in %s on line %d
+Fatal error: Cannot use "parent" when current class scope has no parent in %s on line %d

Zend/zend_compile.c

@@ -1408,7 +1408,7 @@ static void zend_ensure_valid_class_fetch_type(uint32_t fetch_type) /* {{{ */
 				fetch_type == ZEND_FETCH_CLASS_SELF ? "self" :
 				fetch_type == ZEND_FETCH_CLASS_PARENT ? "parent" : "static");
 		} else if (fetch_type == ZEND_FETCH_CLASS_PARENT && !ce->parent_name) {
-			zend_error(E_DEPRECATED,
+			zend_error_noreturn(E_COMPILE_ERROR,
 				"Cannot use \"parent\" when current class scope has no parent");
 		}
 	}