...
Daffodil issues an SDE in this situation as is required by the DFDL spec.
TBD: this SDE may be This SDE is too strong: If the element has IVC, then it doesn't need a value for unparsing unless an expression that is used at unparse time (i.e., not in an assert or discriminator) references the element. In many cases the value of an IVC may be used only in dfdl:occursCount expressions, or in assert/discriminator test expressions. Since those aren't evaluated when unparsing, there is no unparsing situation would never need the value, so the SDE is too strong.
IVC and OVC on the same element
...
In the above, when unparsing, the output value calc for len can be evaluated, but we must delay its evaluation and unparsing until the subsequent data element is available.
The next thing the unparse has to do, after delaying the unparsing of 'len' is set the var variable. This requires the value of len, which has been deferred.
We have real schemas (e.g., even PCAP) where this occurs.
This breaks the rule that when variables are evaluated, things they reference must have already been evaluated. Basically, when we delay evaluating the OVC for len we are suspending anything that depends on len having a value as well.
...