Revert "Detect invalid uses of parent:: during compilation"

This reverts commit a9e6667.

Breakage found in the wild: Mockery uses a parent:: call in the
implementation regardless of whether the class has a parent or not:
https://github.com/mockery/mockery/blob/4324afeaf9d95b492507e6587abb3f024e2576de/library/Mockery/Mock.php#L600

This change is not worth the compat break in 7.4.

Author: nikic

UPGRADING

@@ -22,9 +22,6 @@ PHP 7.4 UPGRADE NOTES
 ========================================
 
 - Core:
-  . Referencing parent:: inside a class that does not have a parent will now
-    generate a compile-time error. Previously the error was only emitted at
-    run-time.
   . get_declared_classes() no longer returns anonymous classes that haven't
     been instantiated yet.
 

Zend/tests/class_name_as_scalar_error_002.phpt

@@ -11,4 +11,7 @@ namespace Foo\Bar {
 }
 ?>
 --EXPECTF--
-Fatal error: 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

Zend/zend_compile.c

@@ -1317,16 +1317,10 @@ static uint32_t zend_get_class_fetch_type_ast(zend_ast *name_ast) /* {{{ */
 
 static void zend_ensure_valid_class_fetch_type(uint32_t fetch_type) /* {{{ */
 {
-	if (fetch_type != ZEND_FETCH_CLASS_DEFAULT && zend_is_scope_known()) {
-		zend_class_entry *ce = CG(active_class_entry);
-		if (!ce) {
-			zend_error_noreturn(E_COMPILE_ERROR, "Cannot use \"%s\" when no class scope is active",
-				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_noreturn(E_COMPILE_ERROR,
-				"Cannot use \"parent\" when current class scope has no parent");
-		}
+	if (fetch_type != ZEND_FETCH_CLASS_DEFAULT && !CG(active_class_entry) && zend_is_scope_known()) {
+		zend_error_noreturn(E_COMPILE_ERROR, "Cannot use \"%s\" when no class scope is active",
+			fetch_type == ZEND_FETCH_CLASS_SELF ? "self" :
+			fetch_type == ZEND_FETCH_CLASS_PARENT ? "parent" : "static");
 	}
 }
 /* }}} */