Record field order in a bidirectional pattern synonym match is order dependent

Consider this example,

{-# LANGUAGE PatternSynonyms #-}
module Test where

data ARecord = ARecord {anInt :: Int, aString :: String}

-- This works...
pattern AGoodPat :: Int -> String -> ARecord
pattern AGoodPat n s = ARecord {anInt=n, aString=s}

-- But if we invert the order of the fields things break...
pattern ABadPat :: Int -> String -> ARecord
pattern ABadPat n s = ARecord {aString=s, anInt=n}

Despite the fact that AGoodPat and ABadPat differ only in the order of the fields in their matches, ABadPat fails to typecheck with,

Test.hs:12:40: error:
    • Couldn't match type ‘[Char]’ with ‘Int’
      Expected type: Int
        Actual type: String
    • In the first argument of ‘ARecord’, namely ‘s’
      In the expression: ARecord s n
      In an equation for ‘$bABadPat’: $bABadPat n s = ARecord s n

Test.hs:12:49: error:
    • Couldn't match type ‘Int’ with ‘[Char]’
      Expected type: String
        Actual type: Int
    • In the second argument of ‘ARecord’, namely ‘n’
      In the expression: ARecord s n
      In an equation for ‘$bABadPat’: $bABadPat n s = ARecord s n

This can be observed with both 7.10.3 and master.

If one makes ABadPat into a one-directional pattern then the error vanishes. The fact that field order matters in the case of bi-directional record pattern synonyms seems a bit odd. If there's a good reason why this must be the case then fair enough, but we should make sure this is mentioned in the users guide and provide a more helpful error message.

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	mpickering
Operating system
Architecture