When the feature added in
PIG-1434, there was a proposal to cast it to tuple, to be able to use as a scalar. But for some reason, in the implementation this cast was not required.
The current syntax which does not need this cast seems to lead to lot of confusion among users, who end up using this feature unintentionally. This usually happens because the user is referring to the bag column(s) in output of (co)group using a wrong name, which happens to be another relation. Often, users realize the mistake only at runtime. New users, will have trouble figuring out what was wrong.
I think we should support the use of the cast as originally proposed, and deprecate the current syntax. The warning issued when the deprecated syntax is used is likely to help users realize that they have unintentionally used this feature.