[GHC] #10946: Typed hole inside typed Template Haskell bracket causes panic

[GHC]

#10946: Typed hole inside typed Template Haskell bracket causes panic
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        Reporter:  jstolarek         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.4.1
       Component:  Template Haskell  |              Version:  7.10.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |            Test Case:  th/T10946
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by elliottt):

 The problem seems to be that the quoted expression {{{[|| _x ||]}}} will
 introduce a hole constraint of the form {{{ _x :: a }}} when spliced, but
 that the error will be reported outside of the implication introduced by
 the signature. When checking the following program:

 {{{
 m :: a
 m  = _x
 }}}

 {{{simplifyTop}}} will report that it's found an unsolved wanted
 constraint of the form {{{ forall (a :: *). () => _x :: a }}}. However,
 when checking the original program, {{{simplifyTop}}} is called from a
 different path ({{{tcTopSpliceExpr}}}), and reports the wanted constraint
 as being of the form {{{ _x :: a }}}. As it lacks the context of the
 binding for {{{a}}}, it falls through to the base case of
 {{{getSkolemInfo}}}, which causes the panic.

 At this point, I'm not sure what the right path forward is. Should
 {{{tcTopSpliceExpr}}} be using {{{simplifyTop}}}, or is there just some
 additional context needed to be setup before that call?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10946#comment:8>
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