Derive synthetic type bounds for expressions

src/typing.h

Index: src/typing.h
diff --git a/src/typing.h b/src/typing.h
index 2d3fac0650959cb9f6ac719b3973456510702b69..a3d48623a6de35e1979620fb37ba76778f42c8b0 100644
--- a/src/typing.h
+++ b/src/typing.h
@@ -68,6 +68,20 @@ class AstTyper: public AstVisitor {
   void MergeUpperType(Expression* e, Handle<Type> t) {
     e->set_upper_type(handle(Type::Intersect(e->upper_type(), t), isolate_));
   }
+  void MergeLowerType(Expression* e, Type* t) {
+    MergeLowerType(e, handle(t, isolate_));
+  }
+  void MergeUpperType(Expression* e, Type* t) {
+    MergeUpperType(e, handle(t, isolate_));
+  }
+
+  static Type* BoundedType(Type* bound, Handle<Type> type) {
+    return type->Is(bound) ? *type : bound;
+  }
+  static Type* AdditionType(Handle<Type> type) {
+    return type->Is(Type::Number()) ? Type::Number() :
+           type->Is(Type::String()) ? Type::String() : Type::NumberOrString();
+  }
 
   void VisitDeclarations(ZoneList<Declaration*>* declarations);
   void VisitStatements(ZoneList<Statement*>* statements);