GHC wrongly warns of incomplete pattern matches when using pattern synonyms.

If one writes the following code:

{-# LANGUAGE PatternSynonyms #-}
module Version where
newtype Version = Version [Integer]
    deriving (Eq)
pattern V :: [Integer] -> Version
pattern V is = Version is
instance Ord Version where
    (V []) <= (V []) = True
    (V []) <= _ = True
    _ <= (Version []) = False
    (V (x:xs)) <= (V (y:ys))
        | x == y = Version xs <= Version ys
        | otherwise = x <= y

and compiles with ghc -Wall -Werror, one gets the following message:

[1 of 1] Compiling Version          ( Bug.hs, Bug.o )
Bug.hs:12:5: error: [-Wincomplete-patterns, -Werror=incomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for ‘<=’:
        Patterns not matched:
            _ _
            _ _
            _ _
            _ _
   |
12 |     (V []) <= (V []) = True
   |     ^^^^^^^^^^^^^^^^^^^^^^^...

However, the following code compiles correctly without warnings:

{-# LANGUAGE PatternSynonyms #-}
module Version where
newtype Version = Version [Integer]
    deriving (Eq)
instance Ord Version where
    (Version []) <= (Version []) = True
    (Version []) <= _ = True
    _ <= (Version []) = False
    (Version (x:xs)) <= (Version (y:ys))
        | x == y = Version xs <= Version ys
        | otherwise = x <= y

This bug appears in GHC 8.2.2 as well as GHC-8.4.1-alpha2.

Trac metadata

Trac field	Value
Version	8.4.1-alpha1
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
Test case
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closed

This is expected behaviour: Because pattern synonyms can do arbitrary computations, the compiler does not even attempt to detect when they are complete.

You can specify

{-# COMPLETE V #-}

to declare that the V pattern is complete.

Also see #8779 (closed).

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedDuplicate

added Plow label

added pattern match warnings label

added PatternSynonyms label