+NB: this is not (quite) the same as being a "naughty" record selector
+(See Note [Naughty record selectors]) in TcTyClsDecls), at least
+in the case of GADTs. Consider
+ data T a where { MkT :: { f :: a } :: T [a] }
+Then f is not "naughty" because it has a well-typed record selector.
+But we don't allow updates for 'f'. (One could consider trying to
+allow this, but it makes my head hurt. Badly. And no one has asked
+for it.)
+