[GHC] #13644: overloaded name used in record pattern matching leads to panic! (the 'impossible' happened) in ghc

[GHC]

#13644: overloaded name used in record pattern matching leads to panic!  (the
'impossible' happened) in ghc
-------------------------------------+-------------------------------------
        Reporter:  pjljvdlaar        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.0.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13840, #13973,   |  Differential Rev(s):
  #14087                             |
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by sighingnow):

 Maybe we should throw a type error when pattern name mismatch, rather than
 just panic.

 Use the case from #13973 as an example,

 {{{#!haskell
 -- Record.hs
 module Record (Record(..)) where

 data Record = Record { field1 :: Int, field2 :: Int }

 -- Test.hs
 module Test (foo) where

 import qualified Record as Rec

 field2 :: ()
 field2 = ()

 foo :: Rec.Record -> Int
 foo Rec.Record{field2 = field2} = field2
 }}}

 In function `translateConPatVec` (in `compiler/deSugar/Check.hs`, during
 the desugar pass), we can't know whether the pattern label comes from the
 record fields, or just another ordinary symbol.

 When pattern name lookup (`lookup name zipped`) fails, **we could reach a
 conclusion** that `name` must not be one of the record fields. Then the
 compiler can throw an error like

 {{{
 Test.hs:11:16: error:
     Record field not in scope: ‘field2’
     Perhaps you meant one of these:
       ‘Rec.field1’ (imported from Record),
       ‘Rec.field2’ (imported from Record),
 }}}

 (Noticing that `Record field not in scope` has point out that the missing
 item is a record field label, rather than other ordinary variable, without
 causing any confusion.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13644#comment:14>
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