[GHC] #14093: GHC.Exe Panic (The 'Impossible' happened)

[GHC]

#14093: GHC.Exe Panic (The 'Impossible' happened)
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           Reporter:  ColonelTrick   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Unfortunately, I am just learning Haskell, so I do not have the context to
 narrow this down to a narrow repro.

 Console Output:

 PS C:\Haskell> ghc -o creditCard .\CreditCards.hs
 [1 of 1] Compiling Main             ( CreditCards.hs, Credit
 ghc.exe: panic! (the 'impossible' happened)
   (GHC version 8.0.2 for x86_64-unknown-mingw32):
         initTc: unsolved constraints
   WC {wc_insol = [W] n_a1PN :: t_a1PM[tau:1] (CHoleCan: n)}

 CreditCards.hs

 {{{#!hs
 -- Converts a number to digits
 toDigits :: Integer -> [Integer]

 toDigits n
   | n <= 0 = []
   | otherwise = n `mod` 10 : toDigits (n `div` 10)

 -- Converts a number to digits and then reverses it
 toDigitsRev :: Integer -> [Integer]
 toDigitsRev n = reverse . toDigits n

 -- Doubles every other value
 doubleEveryOther :: [Integer] -> [Integer]

 doubleEveryOther[] = []
 doubleEveryOther ([x:[]]) = [x]
 doubleEveryOther (x : y : zs) = x : 2 * y : doubleEveryOther zs

 -- Sums the digits of the credit card.
 sumDigits :: [Integer] -> [Integer]

 sumDigits [] = 0
 sumDigits (x:xs)
   | x < 10 = x + sumDigits xs
   | otherwise = (x `mod` 10) + (x `div` 10) + sumDigits xs

 --- Inidicates whether the credit card is valid.
 validate :: [Integer] -> Bool

 validate x =  (sumDigits (doubleEveryOther (toDigitsRev n)) `mod` 10) == 0
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14093>
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