[GHC] #14167: GHC often depicts H98 datatypes as GADTs when it shouldn't

[GHC]

#14167: GHC often depicts H98 datatypes as GADTs when it shouldn't
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  GADTs,
                                     |  TemplateHaskell
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D3901
       Wiki Page:                    |
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Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"8e4229ab3dc3e1717ad557ef00f3518da6b5c523/ghc" 8e4229a/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="8e4229ab3dc3e1717ad557ef00f3518da6b5c523"
 Fix #14167 by using isGadtSyntaxTyCon in more places

 Summary:
 Two places in GHC effectively attempt to //guess// whether a data type
 was declared using GADT syntax:

 1. When reifying a data type in Template Haskell
 2. When pretty-printing a data type (e.g., via `:info` in GHCi)

 But there's no need for heuristics here, since we have a 100% accurate way
 to
 determine whether a data type was declared using GADT syntax: the
 `isGadtSyntaxTyCon` function! By simply using that as the metric, we
 obtain
 far more accurate TH reification and pretty-printing results.

 This is technically a breaking change, since Template Haskell reification
 will
 now reify some data type constructors as `(Rec)GadtC` that it didn't
 before,
 and some data type constructors that were previously reified as
 `(Rec)GadtC`
 will no longer be reified as such. But it's a very understandable breaking
 change, since the previous behavior was simply incorrect.

 Test Plan: ./validate

 Reviewers: simonpj, goldfire, austin, bgamari

 Reviewed By: simonpj

 Subscribers: rwbarton, thomie

 GHC Trac Issues: #14167

 Differential Revision: https://phabricator.haskell.org/D3901
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14167#comment:3>
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