Current scope
Operator such as e1 op e2. In practice, either TOK.question or one of the binary operator.
The LHS of the operation, will be rewritten
The RHS of the operation, will be rewritten
The resulting type in case of success, null in case of error
Merge types of e1 and e2 into a common subset
Parameters e1 and e2 will be rewritten in place as needed.