When the deriving strategies extension isn't enabled , what will the new semantics be when more than one strategy applies? What's our new answer there ?
GHC already has a process for resolving which strategy to pick in a plain old deriving statement, but it isn't well documented. I've added a section to the users' guide in [1], but I'll summarize it here. When processing a deriving statement without an explicit strategy: 1. If the typeclass is built-in (or wired-in, as you suggested), and any necessary language extension is enabled (e.g., -XDeriveFunctor for Functor), then GHC will use the corresponding built-in algorithm to derive the instance. 2. Otherwise, GHC checks if either -XGeneralizedNewtypeDeriving or -XDeriveAnyClass are enabled, and if the class is able to be derived with one of those approaches, GHC does so. If BOTH extensions are enabled and the class can be derived with either approach, GHC defaults to -XDeriveAnyClass (but also emitting a warning about the choice it made to resolve the ambiguity). 3. Otherwise, GHC errors. Again, this is current GHC behavior, not a new or proposed feature. Ryan S. ----- [1] https://phabricator.haskell.org/D2280#02e78429