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

frog/leg/compile_time_constants.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.
 
 class Constant implements Hashable {
   const Constant();
 
   bool isNull() => false;
   bool isBool() => false;
   bool isTrue() => false;
   bool isFalse() => false;
   bool isInt() => false;
   bool isDouble() => false;
   bool isNum() => false;
   bool isString() => false;
   bool isList() => false;
   bool isMap() => false;
   bool isConstructedObject() => false;
+  /** Returns true if the constant is null, a bool, a number or a string. */
+  bool isPrimitive() => false;
   /** Returns true if the constant is a list, a map or a constructed object. */
   bool isObject() => false;
 
   abstract void writeJsCode(StringBuffer buffer, ConstantHandler handler);
   abstract List<Constant> getDependencies();
 }
 
 class PrimitiveConstant extends Constant {
   abstract get value();
   const PrimitiveConstant();
+  bool isPrimitive() => true;
 
   bool operator ==(var other) {
     if (other is !PrimitiveConstant) return false;
     PrimitiveConstant otherPrimitive = other;
     // We use == instead of === so that DartStrings compare correctly.
     return value == otherPrimitive.value;
   }
 
   String toString() => value.toString();
   // Primitive constants don't have dependencies.
@@ skipping 851 matching lines @@
         case "<=":
           folded = const LessEqualOperation().fold(left, right);
           break;
         case ">":
           folded = const GreaterOperation().fold(left, right);
           break;
         case ">=":
           folded = const GreaterEqualOperation().fold(left, right);
           break;
         case "==":
-          folded = const EqualsOperation().fold(left, right);
+          if (left.isPrimitive() && right.isPrimitive()) { 

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

Is "null" a numeric, string or boolean value? It seems null isn't included in
the spec allowances for compile-time constants comparisons.

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

Filed already a bug, and Gilad confirmed that null should work too.

+            folded = const EqualsOperation().fold(left, right);
+          }
           break;
         case "===":
-          folded = const IdentityOperation().fold(left, right);
+          if (left.isPrimitive() && right.isPrimitive()) { 
+            folded = const IdentityOperation().fold(left, right);
+          }
           break;
         case "!=":
-          BoolConstant areEquals = const EqualsOperation().fold(left, right);
-          if (areEquals === null) {
-            folded = null;
-          } else {
-            folded = areEquals.negate();
+          if (left.isPrimitive() && right.isPrimitive()) { 
+            BoolConstant areEquals = const EqualsOperation().fold(left, right);
+            if (areEquals === null) {
+              folded = null;
+            } else {
+              folded = areEquals.negate();
+            }
           }
           break;
         case "!==":
-          BoolConstant areIdentical =
-              const IdentityOperation().fold(left, right);
-          if (areIdentical === null) {
-            folded = null;
-          } else {
-            folded = areIdentical.negate();
+          if (left.isPrimitive() && right.isPrimitive()) { 
+            BoolConstant areIdentical =
+                const IdentityOperation().fold(left, right);
+            if (areIdentical === null) {

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

Have you considered having a dedicated FoldingFailed value you can return
instead of null. It would be infectous, so any operation using it would itself
be a failure (e.g., call it NaCTC :), and you could avoid null checks like this.

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

I have considered it, but the problem is, that you want nice error-messages.
This means that there would need to be a way to attach a node to a Constant then
(since we don't want to work conditionally) which seems not so nice.
That said I'm not convinced that this is the cleanest it can get. I just think
that currently it is good enough and not too bad.

+              folded = null;
+            } else {
+              folded = areIdentical.negate();
+            }
           }
           break;
         default:
           compiler.internalError("Unexpected operator.", node: op);
           break;
       }
       if (folded === null) error(send);
       return folded;
     }
     return super.visitSend(send);
@@ skipping 130 matching lines @@
     return node.expression.accept(this);
   }
 
   error(Node node) {
     // TODO(floitsch): get the list of constants that are currently compiled
     // and present some kind of stack-trace.
     MessageKind kind = MessageKind.NOT_A_COMPILE_TIME_CONSTANT;
     compiler.reportError(node, new CompileTimeConstantError(kind, const []));
   }
 }