[GHC] #11345: Template Haskell's handling of infix GADT constructors is broken

[GHC]

#11345: Template Haskell's handling of infix GADT constructors is broken
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        Reporter:  RyanGlScott       |                Owner:  RyanGlScott
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Template Haskell  |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect result  |  Unknown/Multiple
  at runtime                         |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1744
       Wiki Page:                    |
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Comment (by jstolarek):

 I finally had a moment to take a closer look at the code and I have some
 concerns. Most importantly, I don't understand how we can actually have
 infix GADT constructors. Splicing creates an `HsDecl.ConDecl` data type,
 which can be either `ConDeclGADT` or `ConDeclH98`. The latter constructor
 has field `con_details :: HsConDeclDetails name`, where `HsConDeclDetails`
 is a type synonym to:

 {{{#!hs
 data HsConDetails arg rec
   = PrefixCon [arg]             -- C p1 p2 p3
   | RecCon    rec               -- C { x = p1, y = p2 }
   | InfixCon  arg arg           -- p1 `C` p2
 }}}
 There is no such field in `ConDeclGADT` data constructor, so how can we
 mark a constructor declaration as infix? You're asking:

 > what happens when you try to splice an infix `GadtC` into source code in
 `Convert.hs`?

 Indeed, what should happen in such a situation? I believe we should splice
 a normal GADT constructor because we have no way of expressing the
 infixity. Ryan, you also said in an earlier comment:

 > A GADT constructor is only considered to be declared infix if (a) it is
 an operator symbol, (b) it has two arguments, (c) it has a fixity
 declaration.

 Can you show me where this happens in code?

 Your ticket reports actual problems in the implementation but I am not
 sure that the way you trying to address them is correct.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11345#comment:7>
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