Potentially a bit out of scope for the pitch here, but still an important question nonetheless! Currently diagnostics are our main tool here, and in fact this is one of the compelling reasons why we'd like to use macros instead of a solution like operator overloading. The macro will produce diagnostics for this case that tell the developer that the function is not able to be used in the context of the predicate they are creating. For common functions, we also have the opportunity to provide fix-its or suggestions as applicable within these diagnostics, but in general the diagnostic will just alert the developer that this function can't be used. Currently, our main source of truth for developers to see a list of supported operations would be the documentation (for example, predicates support all expressions that conform to StandardPredicateExpression). Macros don't currently have a way to influence the autocomplete results, but that is an avenue that could be interesting to bring up on the macro proposal.
2 Likes