Error out on compile-time constants if equality is tested on bad types.

frog/leg/operations.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 interface Operation {
   
 }
 
 interface UnaryOperation extends Operation {
   /** Returns [:null:] if it was unable to fold the operation. */
@@ skipping 223 matching lines @@
 
 class EqualsOperation implements BinaryOperation {
   const EqualsOperation();
   Constant fold(Constant left, Constant right) {
     if (left.isNum() && right.isNum()) {
       // Numbers need to be treated specially because: NaN != NaN, -0.0 == 0.0,
       // and 1 == 1.0.
       NumConstant leftNum = left;
       NumConstant rightNum = right;
       return new BoolConstant(leftNum.value == rightNum.value);
+    } else if (!left.isConstructedObject()) {

[Lasse Reichstein Nielsen, 2012/03/27 08:12:12]

I'd prefer 
  else if (left.isConstructedObject()) {
    return null;
  } else {
    ...
instead of the negative test. 
Generally, negative tests in if/else-if chains reduces readability *a lot*.

You can also drop the "else", since each branch returns.

[floitsch, 2012/03/28 00:25:21]

hehe. that's what I had before. After long hesitation I move the 'return null'
to the end. Reverted.
done.

+      // Unless we know that the user-defined object does not implement the
+      // equality operator we cannot fold here.
+      return new BoolConstant(left == right);
     }
-    return new BoolConstant(left == right);
+    return null;
   }
 }
 
 class IdentityOperation implements BinaryOperation {
   const IdentityOperation();
   Constant fold(Constant left, Constant right) {
     return new BoolConstant(left == right);
   }
 }