This introduces the minimal changes for
FunTuple to understand the optional return type of
Note that Rec!2977 (merged) does not yet change the behavior of e.g.
MapRelInputToFunOuput. These Functors still return their
My proposal would be to merge Rec!2977 (merged) as is + this MR.
Then I can create a follow-up MR that removes the current usage of the
constexpr invalid_value member to replace it with the new optional return type functionality.
But for that we also need to adapt
FunTuple with the features to let us specify a default invalid value for a specific column in the tuple.
@amathad I am hoping you might have time to help with the latter? I already looked at
FunTuple.cpp but wasn't sure where this should best be implemented.