Fix GH-9769: Improve error message for array unpacking in constant expression

Zend/tests/array_unpack/gh9769.phpt

@@ -0,0 +1,15 @@
+--TEST--
+Unpacking arrays in constant expression
+--FILE--
+<?php
+
+const A = [...[1, 2, 3]];
+const B = [...['a'=>1, 'b'=>2, 'c'=>3]];
+const C = [...new ArrayObject()];
+
+?>
+--EXPECTF--
+Fatal error: Uncaught Error: Only arrays can be unpacked in constant expression in %sgh9769.php:5
+Stack trace:
+#0 {main}
+  thrown in %sgh9769.php on line 5

Zend/zend_ast.c

@@ -475,8 +475,7 @@ static zend_result zend_ast_add_unpacked_element(zval *result, zval *expr) {
 		return SUCCESS;
 	}
 
-	/* Objects or references cannot occur in a constant expression. */
-	zend_throw_error(NULL, "Only arrays and Traversables can be unpacked");
+	zend_throw_error(NULL, "Only arrays can be unpacked in constant expression");
 	return FAILURE;
 }