[GHC] #12928: Too easy to trigger CUSK condition using TH

[GHC]

#12928: Too easy to trigger CUSK condition using TH
-------------------------------------+-------------------------------------
           Reporter:  int-index      |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  high           |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 While trying to make `singletons` class promotion work with kind-
 polymorphic type variables, I've encountered a problem regarding CUSKs.

 Consider this class:

 {{{
 $(promoteOnly [d|
   class Cls a  where
     fff :: Proxy a -> ()
   |])
 }}}

 The generated TH (slightly simplified by hand) looks like this:

 {{{
 type FffSym1 (t_alvm :: Proxy a1627472612) = Fff t_alvm

 data FffSym0 (l_alvn :: TyFun (Proxy a1627472612) ())

 type instance Apply FffSym0 l_alvn = FffSym1 l_alvn

 class PCls a_alve where
   type Fff (arg_alvl :: Proxy a_alve) :: ()
 }}}

 However, it fails to compile with the following error:

 {{{
 cusk.hs:25:1: error:
     You have written a *complete user-suppled kind signature*,
     but the following variable is undetermined: k0 :: *
     Perhaps add a kind signature.
     Inferred kinds of user-written variables:
       a1627472612 :: k0
       l_alvn :: TyFun (Proxy a1627472612) ()
 }}}

 As suggested by the error message, we need to specify the kind in its
 entirety (write a proper CUSK). So to sidestep the issue, we can write

 {{{
 data FffSym0 (l_alvn :: TyFun (Proxy (a1627472612 :: k)) ())
 }}}

 and it'll work. However, this means that the TH code generator should
 perform some guesswork to modify kind annotations. It sounds like a
 minefield — too much can go wrong, so it'd be great to avoid doing so.

 Looking at the module `TcHsType` in the GHC sources, I see that the error
 is reported by the `kcHsTyVarBndrs` function:

 {{{
           -- If there are any meta-tvs left, the user has
           -- lied about having a CUSK. Error.
        ; let (meta_tvs, good_tvs) = partition isMetaTyVar qkvs
        ; when (not (null meta_tvs)) $
          report_non_cusk_tvs (qkvs ++ tc_tvs)
 }}}

 However, I don't see why we can't generalize over `meta_tvs` instead of
 reporting a failure. I asked on IRC #ghc and ezyang suggested that it
 should be sound.

 Below I attach a self-contained example that demonstrates the issue and
 has no dependencies except `base`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12928>
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