From 59fdb656f09cb4f51fc60d30d8c1bef59f5f908d Mon Sep 17 00:00:00 2001
From: Dan Liew <daniel.liew@imperial.ac.uk>
Date: Thu, 12 Feb 2015 11:51:20 +0000
Subject: Protect Bitvector field of BvExtractExpr when it is immutable.

---
 Source/Core/AbsyExpr.cs                        | 18 ++++++++++++++----
 Source/UnitTests/CoreTests/ExprImmutability.cs |  9 +++++++++
 2 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/Source/Core/AbsyExpr.cs b/Source/Core/AbsyExpr.cs
index 2427d856..f3a943b8 100644
--- a/Source/Core/AbsyExpr.cs
+++ b/Source/Core/AbsyExpr.cs
@@ -2969,11 +2969,21 @@ namespace Microsoft.Boogie {
   }
 
   public class BvExtractExpr : Expr {
-    // FIXME: Protect these fields
-    public /*readonly--except in StandardVisitor*/ Expr/*!*/ Bitvector;
+    private /*readonly--except in StandardVisitor*/ Expr/*!*/ _Bitvector;
+    public Expr Bitvector {
+      get { 
+        return _Bitvector;
+      }
+      set {
+        if (Immutable)
+          throw new InvalidOperationException("Cannot change BitVector field of an immutable BvExtractExpr");
+        
+        _Bitvector = value;
+      }
+    }
     [ContractInvariantMethod]
     void ObjectInvariant() {
-      Contract.Invariant(Bitvector != null);
+      Contract.Invariant(_Bitvector != null);
     }
 
     public readonly int Start, End;
@@ -2982,7 +2992,7 @@ namespace Microsoft.Boogie {
       : base(tok, immutable) {
       Contract.Requires(tok != null);
       Contract.Requires(bv != null);
-      Bitvector = bv;
+      _Bitvector = bv;
       Start = start;
       End = end;
       if (immutable)
diff --git a/Source/UnitTests/CoreTests/ExprImmutability.cs b/Source/UnitTests/CoreTests/ExprImmutability.cs
index 98e7158f..b83983b6 100644
--- a/Source/UnitTests/CoreTests/ExprImmutability.cs
+++ b/Source/UnitTests/CoreTests/ExprImmutability.cs
@@ -269,6 +269,15 @@ namespace CoreTests
             Assert.IsTrue(concat.Immutable);
             concat.E1 = lhs; // Should throw
         }
+
+        [Test(), ExpectedException(typeof(InvalidOperationException))]
+        public void ProtectedBvExtract()
+        {
+            var bv = new LiteralExpr(Token.NoToken, Microsoft.Basetypes.BigNum.FromInt(0), 32, /*immutable=*/true);
+            var extract = new BvExtractExpr(Token.NoToken, bv, 32, 0, /*immutable=*/true);
+            Assert.IsTrue(extract.Immutable);
+            extract.Bitvector = bv; // Should throw
+        }
     }
 }
 
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