legal casts

[jnfoster]

Currently we write:

A struct/header can be cast to another struct/header if and only if they have the same number of fields and all the fields in the source can be recursively cast to the corresponding fields in the destination.

But we allow tuples and lists to be assigned to struct/headers. So it seems that this should be generalized to struct/header/tuple/lists too?

FWIW, the implementation is stricter than I'd expect as a programmer:
p4lang/p4c#529

[mihaibudiu]

I think that this claim is not implemented as described in the compiler.
In fact, the fields have to be exactly the same types.

Indeed, the type of a ListExpression in the compiler is a tuple, and we allow tuples to be assigned to structs and headers. We have an open issue about assigning lists to headers.

So this paragraph has to be expanded.

[jnfoster]

Indeed. Now we have a placeholder so we don't forget to address this.

[mihaibudiu]

We should assign these issues to people to make sure they are covered.

[mihaibudiu]

We should also forbid casts that do not involve only base types.