You *can* put `LL` into a COMPLETE set, but under the stipulation that you specify which type constructor it's monomorphized to. To quote the wiki page on COMPLETE sets: In the case where all the patterns are polymorphic, a user must provide a type signature but we accept the definition regardless of the type signature they provide. The type constructor for the whole set of patterns is the type constructor as specified by the user. If the user does not provide a type signature then the definition is rejected as ambiguous. This design is a consequence of the design of the pattern match checker. Complete sets of patterns must be identified relative to a type. This is a sanity check as users would never be able to match on all constructors if the set of patterns is inconsistent in this manner. In other words, this would work provided that you'd be willing to list every single instance of `HasSrcSpan` in its own COMPLETE set. It's tedious, but possible. Ryan S. ----- [1] https://ghc.haskell.org/trac/ghc/wiki/PatternSynonyms/CompleteSigs#Typing