[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 RyanGlScott):

 Replying to [comment:7 jstolarek]:
 > There is no such field in `ConDeclGADT` data constructor, so how can we
 mark a constructor declaration as infix?

 This was my mistake. I thought that infix GADT constructors were marked
 with their own constructor ''à la'' `InfixCon`, but that turns out not to
 be the case. Instead, GHC simply checks for three properties in a
 generalized `PrefixCon`.

 > > 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.

 Luckily, we don't need to express the infixity directly in the `GadtC`, as
 it suffices to splice in a `GadtC` with a symbol name and exactly two
 arguments, plus a separate fixity declaration. GHC takses care of the rest
 (see the test case in Phab:D1744 for proof).

 > > 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?

 These are checked via
 [https://git.haskell.org/ghc.git/blob/c78fedde7055490ca6f6210ada797190f3c35d87:/compiler/typecheck/TcTyClsDecls.hs#l1469
 tcConIsInfixGADT].

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11345#comment:8>
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