[GHC] #11667: Incorrect pattern synonym types in error messages

[GHC]

#11667: Incorrect pattern synonym types in error messages
-------------------------------------+-------------------------------------
        Reporter:  rdragon           |                Owner:  rdragon
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.2
       Component:  Compiler          |              Version:  8.0.1-rc2
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11213            |  Differential Rev(s):  Phab:D1967
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Ben Gamari <ben@…>):

 In [changeset:"997312b04c56017197250096d61f3e8fab502319/ghc" 997312b/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="997312b04c56017197250096d61f3e8fab502319"
 Add `PatSynSigSkol` and modify `PatSynCtxt`

 As the type of a pattern synonym cannot in general be represented by a
 value of type Type, we cannot use a value `SigSkol (PatSynCtxt n) (Check
 ty)` to represent the signature of a pattern synonym (this causes
 incorrect signatures to be printed in error messages). Therefore we now
 represent it by a value `PatSynSigSkol n` (instead of incorrect
 signatures we simply print no explicit signature).

 Furthermore, we rename `PatSynCtxt` to `PatSynBuilderCtxt`, and use
 `SigSkol (PatSynBuilderCtxt n) (Check ty)` to represent the type of a
 bidirectional pattern synonym when used in an expression context.
 Before, this type was represented by a value `SigSkol (PatSynCtxt n)
 (Check ty)`, which caused incorrect error messages.

 Also, in `mk_dict_err` of `typecheck\TcErrors.hs` we now distinguish
 between all enclosing implications and "useful" enclosing implications,
 for better error messages concerning pattern synonyms. See `Note [Useful
 implications]`.

 See the Phabricator page for examples.

 Reviewers: mpickering, goldfire, simonpj, austin, bgamari

 Reviewed By: bgamari

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D1967

 GHC Trac Issues: #11667
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11667#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler