[GHC] #15321: Typed holes in Template Haskell splices produce bewildering error messages

[GHC]

#15321: Typed holes in Template Haskell splices produce bewildering error messages
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Template Haskell  |              Version:  8.4.3
      Resolution:                    |             Keywords:  TypedHoles
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by simonpj):

 * differential:  Phab:D4907 =>


Comment:

 You're right.  In `TcHoleErrors` we see
 {{{
                 lookup :: Either Id GlobalRdrElt -> TcM (Maybe Id)
                 lookup (Left id) = return $ Just id
                 lookup (Right el) =
                   do { thing <- tcLookup (gre_name el)
                      ; case thing of
 }}}
 That call to `tcLookup` checks for staging errors (it calls
 `tcLookupGlobal` which calls `notFound`).

 Easiest solution: something like
 {{{
                 lookup (Right el) =
                   tryTcDiscardingErrs (return Nothing) $
                   do { thing <- tcLookup (gre_name el)
                      ; case thing of
 }}}
 The `tryTcDiscardingErrs` does what it sounds like.

 It's not super-efficient, but it doesn't have to be ... this is in error-
 message generation only.

 It needs a `Note` to explain why the lookup can fail.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15321#comment:4>
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