COMPLETE pragma for MonthOfYear type is questionable.

[Oleg Grenrus]

An example:

{-# OPTIONS_GHC -Wall #-}
import Data.Time.Calendar

ex1 :: Int -> Bool
ex1 1 = True
ex1 2 = True
ex1 3 = True
ex1 4 = True
ex1 5 = True
ex1 6 = True
ex1 7 = True
ex1 8 = True
ex1 9 = True
ex1 10 = True
ex1 11 = True
ex1 12 = True

ex2 :: Int -> Bool
ex2 January   = True
ex2 February  = True
ex2 March     = True
ex2 April     = True
ex2 May       = True
ex2 June      = True
ex2 July      = True
ex2 August    = True
ex2 September = True
ex2 October   = True
ex2 November  = True
ex2 December  = True

Warns about ex1 but not about ex2:

Questionable.hs:5:1: warning: [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for ‘ex1’:
        Patterns not matched:
            p where p is not one of {12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1}
  |
5 | ex1 1 = True
  | ^^^^^^^^^^^^...

as type MonthOfYear = Int this is very questionable. Pattern synonyms itself are fine, but claiming that they are complete for Int, without any help even from a newtype wrapper is IMHO not a good idea.

[Ashley Yakeley]

This is intended behaviour.

[Oleg Grenrus]

I don't agree this is great, as MonthOfYear is just Int and there is no (new)type safety net to track provenance of the values, but I won't insist.

[Tom Sydney Kerckhove]

Intended as this may be be, I don't agree it's great either and just wanted to record that here.