[GHC] #12957: In a record-update construct:ghc-stage2: panic! (the 'impossible' happened)

[GHC]

#12957: In a record-update construct:ghc-stage2: panic! (the 'impossible' happened)
-------------------------------------+-------------------------------------
        Reporter:  niteria           |                Owner:  mpickering
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D3065,
       Wiki Page:                    |  Phab:D3064
-------------------------------------+-------------------------------------
Changes (by mpickering):

 * differential:   => Phab:D3065, Phab:D3064


Comment:

 There was actually a bug in the PM checker which was causing this bug to
 show up.

 Here is a simpler reproduction. Essentially the checker was seeing that
 `[]` was not consistent with `(_:_)` and thus for any nested pattern
 matches the initial uncovered set was empty which caused all patterns to
 be reported as redundant.

 {{{
 module T12957 where

 data A = N | A { b :: Bool }
 f = case [] of (_:_) -> case () of
                           a -> undefined

 }}}

 However, it was also good to add the case to `matchSeparator` as these
 matches are printed when `-ddump-ec-trace` is used.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12957#comment:7>
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