This trait takes three arguments: lhs, rhs, transformer. It verifies that the type of 'rhs' matches the type of 'lhs' when the given 'transformer' is applied to 'lhs'. This allows for adding constraints like: "the type of 'a' must match the element type of 'b'". A followup revision will add support in the declarative parser for using these equality constraints to port more c++ parsers to the declarative form.
Depends On D74646