From 733eab5805b36f419c4d466756399e8d01fa9773 Mon Sep 17 00:00:00 2001
From: Matthew Flatt <mflatt@racket-lang.org>
Date: Mon, 28 Feb 2011 15:19:20 -0700
Subject: [PATCH] fix 0-field prefab immutability flag  Closes PR 11777

---
 collects/tests/racket/read.rktl | 3 +++
 src/racket/src/struct.c         | 2 +-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/collects/tests/racket/read.rktl b/collects/tests/racket/read.rktl
index 3ccbb96391..58b83d611f 100644
--- a/collects/tests/racket/read.rktl
+++ b/collects/tests/racket/read.rktl
@@ -1052,6 +1052,7 @@
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Prefab
 
+(test #t struct? (readstr "#s(v)"))
 (test #t struct? (readstr "#s(v 1)"))
 (test #t struct? (readstr "#s((v 1) 1)"))
 (test #t struct? (readstr "#s((v 1 #()) 1)"))
@@ -1078,6 +1079,8 @@
                (and (exn:fail:read? x)
                     (not (exn:fail:read:eof? x)))))
 
+(test #t struct? (syntax-e (read-syntax 'string (open-input-string "#s(v)"))))
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; read-language
 
diff --git a/src/racket/src/struct.c b/src/racket/src/struct.c
index 57087a7abc..c1fd60f9b4 100644
--- a/src/racket/src/struct.c
+++ b/src/racket/src/struct.c
@@ -3929,7 +3929,7 @@ static void struct_type_set_if_immutable(Scheme_Struct_Type *struct_type) {
     size = struct_type->num_islots;
     if (struct_type->name_pos)
       size -= struct_type->parent_types[struct_type->name_pos - 1]->num_islots;
-    if (struct_type->immutables) {
+    if (!size || struct_type->immutables) {
       for (i = 0; i < size; i++) {
         if (!struct_type->immutables[i])
           return;