[GHC] #16077: AvailTC Invariant being violated

[GHC]

#16077: AvailTC Invariant being violated
-------------------------------------+-------------------------------------
        Reporter:  harpocrates       |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |  https://gitlab.haskell.org/ghc/ghc/merge_requests/276
-------------------------------------+-------------------------------------

Comment (by Alec Theriault <alec.theriault@…>):

 In [changeset:"2a431640d199b164ca5d233684d4f4bdaf5ca021/ghc" 2a43164/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="2a431640d199b164ca5d233684d4f4bdaf5ca021"
 Uphold AvailTC Invariant for associated data fams

 The AvailTC was not be upheld for explicit export module
 export lists when the module contains associated data families.

     module A (module A) where
     class    C a  where { data T a }
     instance C () where { data T () = D }

 Used to (incorrectly) report avails as `[C{C, T;}, T{D;}]`. Note that
 although `T` is exported, the avail where it is the parent does _not_
 list it as its first element. This avail is now correctly listed as
 `[C{C, T;}, T{T, D;}]`.

 This was induces a [crash in Haddock][0].

 See #16077.

 [0]: https://github.com/haskell/haddock/issues/979
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16077#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler