[GHC] #9580: Possible excessive leniency in interaction between coerce and data families?

[GHC]

#9580: Possible excessive leniency in interaction between coerce and data
families?
-------------------------------------+-------------------------------------
              Reporter:  dmcclean    |            Owner:  simonpj
                  Type:  bug         |           Status:  new
              Priority:  low         |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
  (Type checker)                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------

Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"0aaf812ed0a4a4be9528b2e2f6b72bee7cd8002d/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="0aaf812ed0a4a4be9528b2e2f6b72bee7cd8002d"
 Clean up Coercible handling, and interaction of data families with
 newtypes

 This patch fixes Trac #9580, in which the Coercible machinery succeeded
 even though the relevant data constructor was not in scope.

 As usual I got dragged into a raft of refactoring changes,
 all for the better.

 * Delete TcEvidence.coercionToTcCoercion (now unused)

 * Move instNewTyConTF_maybe, instNewTyCon_maybe to FamInst,
   and rename them to tcInstNewTyConTF_maybe, tcInstNewTyCon
   (They both return TcCoercions.)

 * tcInstNewTyConTF_maybe also gets more convenient type,
   which improves TcInteract.getCoercibleInst

 * Define FamInst.tcLookupDataFamInst, and use it in TcDeriv,
   (as well as in tcInstNewTyConTF_maybe)

 * Improve error report for Coercible errors, when data familes
   are involved  Another use of tcLookupDataFamInst

 * In TcExpr.tcTagToEnum, use tcLookupDataFamInst to replace
   local hacky code

 * Fix Coercion.instNewTyCon_maybe and Type.newTyConInstRhs to deal
   with eta-reduced newtypes, using
   (new) Type.unwrapNewTyConEtad_maybe and (new) Type.applyTysX

 Some small refactoring of TcSMonad.matchFam.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9580#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler