[GHC Proposal #425 "Invisible binders in type declarations"](https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0425-decl-invis-binders.rst) forbids implicit quantification of type variables that occur free on the right-hand side of a type synonym but are not mentioned on the left-hand side. The users are expected to rewrite this using invisible binders: ```haskell type T1 = 'Nothing :: Maybe a -- old type T1 @a = 'Nothing :: Maybe a -- new ``` Since the `@k`-binders are a new feature, we need to wait for three releases before we require the use of the new syntax. In the meantime, we ought to provide users with a new warning, `-Wimplicit-rhs-quantification`, that would detect when such implicit quantification takes place, and include it in `-Wcompat`. This ticket is to track the implementation of the warning.