[GHC] #13777: Poor error message around CUSKs

[GHC]

#13777: Poor error message around CUSKs
-------------------------------------+-------------------------------------
           Reporter:  goldfire       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1-rc2
           Keywords:  TypeInType,    |  Operating System:  Unknown/Multiple
  CUSKs                              |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 While typing up comment:7:ticket:13761, I came across a poor error message
 around CUSKs.

 {{{
 data Proxy (a :: k) = P
 data S :: forall k. Proxy k -> Type where
   MkS :: S (P :: Proxy Maybe)
 }}}

 produces

 {{{
     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:
 }}}

 That promised list of the kinds of user-written variables is empty. Either
 GHC should find something to print (like `k :: k0`, perhaps) or omit the
 header.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13777>
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