Protect the NAryExpr.Fun field when the NAryExpr is immutable.

Add a unit test for this. We need to protect the Args field too
but that is going to be much harder to enforce.

1 files changed, 7 insertions, 0 deletions

diff --git a/Source/UnitTests/CoreTests/ExprImmutability.cs b/Source/UnitTests/CoreTests/ExprImmutability.cs
index e13feca7..aff0acea 100644
--- a/Source/UnitTests/CoreTests/ExprImmutability.cs
+++ b/Source/UnitTests/CoreTests/ExprImmutability.cs
@@ -96,6 +96,13 @@ namespace CoreTests
             var e = new OldExpr(Token.NoToken, Expr.True, /*immutable=*/ true);
             e.Expr = Expr.False;
         }
+
+        [Test(), ExpectedException(typeof(InvalidOperationException))]
+        public void ProtectedNAryFunc()
+        {
+            var e = GetUnTypedImmutableNAry();
+            e.Fun = new BinaryOperator(Token.NoToken, BinaryOperator.Opcode.Sub);
+        }
     }
 }