[GHC] #16077: AvailTC Invariant being violated

20 Dec 2018

      #16077: AvailTC Invariant being violated
-------------------------------------+-------------------------------------
           Reporter:  harpocrates    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           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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 The `AvailTC Invariant` (from `basicTypes/Avail.hs`) states that:

 {{{
 If the type or class is itself
 to be in scope, it must be
 *first* in this list.  Thus,
 typically: @AvailTC Eq [Eq, ==, \/=]@
 }}}

 Here is a case where this invariant is not upheld:

  * `pkgA/pkgA.cabal`:
 {{{
 name: pkgA
 version: 1.0.0
 build-type: Simple
 cabal-version: >= 1.2

 library
   exposed-modules: A
   build-depends: base
 }}}

  * `pkgA/A.hs`
 {{{
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeFamilies #-}
 module A
   ( C(..)
   , T(TI)
   , error
   ) where

 class C a where
   data T a

 instance C Int where
   data T Int = TI { ti :: Int }
 }}}

  * `pkgB/pkgB.cabal`
 {{{
 name: pkgB
 version: 1.0.0
 build-type: Simple
 cabal-version: >= 1.2

 library
   exposed-modules: B
   build-depends: base, pkgA
 }}}

  * `pkgB/B.hs`
 {{{
 module B
   ( module A
   ) where

 import A hiding (error)
 }}}

 Now, check out the exports for `A` vs. `B`:

 {{{
 $ cabal new-build pkgA pkgB --ghc-options -ddump-rn-trace | grep rnExports
 Warning: The package list for 'hackage.haskell.org' is 38 days old.
 Run 'cabal update' to get the latest list of available packages.
 rnExports: Exports: [error, C{C, T;}, T{T, TI;}]
 rnExports: Exports: [C{C, T;}, T{TI;}]
 }}}

 Why is `T{TI;}` in the second line not `T{T, TI;}`? Shouldn't the exports
 for `B` be identical to those for `A` (except for `error`)?

 This is causing a crash in Haddock:
 https://github.com/haskell/haddock/issues/979.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16077
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