[GHC] #13649: RebindableSyntax causes type errors when 'fail' is not defined, even if not used.

[GHC]

#13649: RebindableSyntax causes type errors when 'fail' is not defined, even if not
used.
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        Reporter:  AaronFriel        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1-rc1
      Resolution:                    |             Keywords:
                                     |  RebindableSyntax
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by nomeata):

 That is indeed a good question. It seems that the type signature of `fail`
 is ignored completely. This might indicate that the renamer requires
 `fail` to be present, but that the type-checker does not see it. And
 `compiler/deSugar/DsExpr.hs` definitely does not emit a call to `fail` if
 the match can fail.

 In function `tcMonadFailOp` the `fail` part of a bind is only type-checked
 if the pattern can fail, using `isIrrefutableHsPat`.

 Potentially, the local function `getFailFunction` in
 {{{
 rnStmt ctxt rnBody (L loc (BindStmt pat body _ _ _)) thing_inside
 }}}
 could do the same.

 Is that enough pointing-the-way for you to try to fix this?

 If you do, make a good note somewhere explaining the interplay of how the
 renamer, the type checker and the desugarer decide whether to assume a
 call to the `fail` function or not.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13649#comment:3>
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