[GHC] #10079: Coercible solver regression: Couldn't match rep of () with Const () b

[GHC]

#10079: Coercible solver regression: Couldn't match rep of () with Const () b
-------------------------------------+-------------------------------------
        Reporter:  glguy             |                   Owner:  goldfire
            Type:  bug               |                  Status:  new
        Priority:  high              |               Milestone:  7.10.1
       Component:  Compiler (Type    |                 Version:  7.10.1-rc1
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  GHC rejects       |               Test Case:  indexed-
  valid program                      |  types/should_compile/T10079
      Blocked By:                    |                Blocking:
 Related Tickets:  #7788, #8550      |  Differential Revisions:  Phab:D653
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Comment (by simonpj):

 > Before Phab:D653, this case was handled by the `rec_nts` trick -- a
 newtype could be unwrapped only once. The problem with this trick is that
 it makes type inference a little wonky: canonicalization wasn't
 idempotent! For example, canonicalizing `Coercible skolem A` would get you
 `Coercible skolem [A]`.

 I don't agree.  Looking at HEAD, I see
 {{{
 can_eq_nc' rdr_env envs ev ReprEq ty1 _ ty2 ps_ty2
   | Just (co, ty1') <- tcTopNormaliseNewTypeTF_maybe envs rdr_env ty1
   = can_eq_newtype_nc rdr_env ev NotSwapped co ty1 ty1' ty2 ps_ty2
 }}}
 This case simply won't fire on `Coercible skolem [A]`. The HEAD does not
 aggressively unwrap newtypes deep inside types, only immediately under a
 `Coercible`.  And that's what we should do in the new version too.

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10079#comment:24>
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