[GHC] #13530: Horrible error message due to TypeInType

[GHC]

#13530: Horrible error message due to TypeInType
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           Reporter:  simonpj        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Consider this
 {{{
 {-# LANGUAGE MagicHash, UnboxedTuples #-}

 module Foo where

 import GHC.Exts

 g :: Int -> (# Int#, a #)
 g (I# y) = (# y, undefined #)

 f :: Int -> (# Int#, Int# #)
 f x = g x
 }}}
 With GHC 8 we get
 {{{
 Foo.hs:11:7: error:
     • Couldn't match a lifted type with an unlifted type
       Expected type: (# Int#, Int# #)
         Actual type: (# Int#, Int# #)
 }}}
 What a terrible error message!!  It was much better in GHC 7.10:
 {{{
 Foo.hs:11:7:
     Couldn't match kind ‘*’ with ‘#’
     When matching types
       a0 :: *
       Int# :: #
     Expected type: (# Int#, Int# #)
       Actual type: (# Int#, a0 #)
 }}}
 What's going on?

 The constraint solver sees
 {{{
 [W] alpha::TYPE LiftedRep  ~  Int#::TYPE IntRep
 }}}
 So it homogenises the kinds, ''and unifies alpha'' (this did not happen in
 GHC 7.10), thus
 {{{
 alpha := Int# |> TYPE co

 [W] co :: LiftedRep ~ IntRep
 }}}
 Of course the new constraint fails.  But since we have unified alpha, when
 we print out the types are are unifying they both look like `(# Int#, Int#
 #)` (there's a suppressed cast in the second component).

 I'm not sure what to do here.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13530>
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