[GHC] #13180: Confusing error when hs-boot abstract data implemented using synonym

[GHC]

#13180: Confusing error when hs-boot abstract data implemented using synonym
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           Reporter:  ezyang         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  low            |         Milestone:
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:  hs-boot        |  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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 This example is a little goofy (that's why I've marked it low priority),
 but here goes:

 {{{
 -- A.hs-boot
 module A where
 data T
 f :: T -> T

 -- B.hs
 module B(f) where
 import {-# SOURCE #-} A

 -- A.hs
 module A(T, f) where
     import qualified B
     type T = Int
     f :: T -> T
     f x = B.f x
 }}}

 When compiled, we get a very interesting error:

 {{{
 ezyang at sabre:~$ ghc-head --make A.hs -fforce-recomp
 [1 of 3] Compiling A[boot]          ( A.hs-boot, A.o-boot )
 [2 of 3] Compiling B                ( B.hs, B.o )
 [3 of 3] Compiling A                ( A.hs, A.o )

 A.hs-boot:2:1: error:
     Type constructor ‘T’ has conflicting definitions in the module
     and its hs-boot file
     Main module: type T = Int
     Boot file:   abstract T
   |
 2 | data T
   | ^^^^^^

 A.hs-boot:3:1: error:
     Identifier ‘f’ has conflicting definitions in the module
     and its hs-boot file
     Main module: f :: T -> T
     Boot file:   f :: T -> T
     The two types are different
   |
 3 | f :: T -> T
   | ^^^^^^^^^^^
 }}}

 The first error is legitimate, but the second is puzzling. I believe the
 problem has to do with the fact that when we load the hs-boot file for
 checking, we knot-tie it against itself. Thus, the T referenced in "Boot
 file" is the one WITHOUT the type synonym unfolding, while the main module
 *does* have the type synonym unfolding.

 Pretty-printing is horribly confusing in these situations, so it would be
 nice if we could give some better guidance here.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13180>
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