Messages cannot be parsed if Specification contains condition with Message'Size

[rssen]

A message is not parsed by PyRFLX if its specification contains a condition where a comparison with Message'Size is performed.

Consider the following example:

package Test is

   type T is mod 2**8;

   type Test is
      message
         A : T
            then B
               with Size => A * 8;
         B : Opaque
            then null
               if Message'Size = (A + 1) * 8;
      end message;
end Test;

It think the problem is that the expression (Message'Size = (A + 1) * 8) is not completely simplified into a BooleanTrue/BooleanFalse by __simplified(l.condition) inside the _next_field method.
That leads to a comparison between an expression.Equal and an expression.BooleanTrue inside the __eq__ method of expression.Expr, which returns NotImplemented.
The core problem is that Message'Size is not substituted with a concrete value (not in __type_literals nor in _simplified_mapping).

As no outgoing condition can be satisfied, the _next_field method returns an empty String, which leads to a KeyError when attempting to get the next field from _fields in the parse method.
The raised KeyError also contains no proper error message, only an empty String.

[jklmnn]

The core problem is that Message'Size is not substituted with a concrete value (not in __type_literals nor in _simplified_mapping).

Message'Size needs to be substituted with the sum of the size of all fields on this path. If there are multiple paths we have to create a disjunction of multiple subsitutions. In this concrete example the result needs to be Message'Size = A'Size + B'Size which then should be resolved to A'Size + B'SIze = (A + 1) * 8.