Allow Throwable to be extended outside of Exception

Zend/tests/bug_test.phpt

@@ -0,0 +1,44 @@
+--TEST--
+Test extending \Throwable directly
+--FILE--
+<?php
+
+class MyThrowable implements Throwable {
+    public function getMessage(): string {
+        return "MyThrowable message";
+    }
+
+    public function getCode() {
+        return 0;
+    }
+
+    public function getFile(): string {
+        return __FILE__;
+    }
+
+    public function getLine(): int {
+        return __LINE__;
+    }
+
+    public function getTrace(): array {
+        return [];
+    }
+
+    public function getPrevious(): ?Throwable {
+        return null;
+    }
+
+    public function getTraceAsString(): string {
+        return "";
+    }
+}
+
+try {
+    throw new MyThrowable();
+} catch (Throwable $e) {
+    echo $e->getMessage(), "\n";
+}
+
+?>
+--EXPECT--
+MyThrowable message

Zend/zend_exceptions.c

@@ -57,27 +57,7 @@ static zend_object_handlers default_exception_handlers;
 /* {{{ zend_implement_throwable */
 static int zend_implement_throwable(zend_class_entry *interface, zend_class_entry *class_type)
 {
-	/* zend_ce_exception and zend_ce_error may not be initialized yet when this is called (e.g when
-	 * implementing Throwable for Exception itself). Perform a manual inheritance check. */
-	zend_class_entry *root = class_type;
-	while (root->parent) {
-		root = root->parent;
-	}
-	if (zend_string_equals_literal(root->name, "Exception")
-			|| zend_string_equals_literal(root->name, "Error")) {
-		return SUCCESS;
-	}
-
-	bool can_extend = (class_type->ce_flags & ZEND_ACC_ENUM) == 0;
-
-	zend_error_noreturn(E_ERROR,
-		can_extend
-			? "%s %s cannot implement interface %s, extend Exception or Error instead"
-			: "%s %s cannot implement interface %s",
-		zend_get_object_type_uc(class_type),
-		ZSTR_VAL(class_type->name),
-		ZSTR_VAL(interface->name));
-	return FAILURE;
+	return SUCCESS;
 }
 /* }}} */