GHC.Exe Panic (The 'Impossible' happened)

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

-- 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

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import labels

closed

Thanks for the bug report. This is a duplicate of #13106 (closed), which has been fixed in GHC 8.2.1. (On GHC 8.2.1, it would give a proper error that n is out of scope in the definition of validate.)

Trac metadata

Trac field	Value
Version	8.2.1 → 8.0.2
Resolution	Unresolved → ResolvedDuplicate
Related	- → #13106 (closed)

added Pnormal label