While using Data.Aeson.TH, "Irrefutable pattern failed for pattern sel_id : _"

changed weight to 5

Attached file Bad.hs ($1239).

I can reproduce the error under Stack LTS-7.5, so this is with aeson-0.11.2.1.

Here's a version which doesn't require installing aeson:

-- Bug2.hs
module Bug2 where

import Language.Haskell.TH

data Options = Options
    { fieldLabelModifier :: String -> String
    , constructorTagModifier :: String -> String
    , allNullaryToStringTag :: Bool
    , omitNothingFields :: Bool
    , sumEncoding :: SumEncoding
    , unwrapUnaryRecords :: Bool
    }

data SumEncoding =
    TaggedObject { tagFieldName      :: String
                 , contentsFieldName :: String
                 }
  | ObjectWithSingleField
  | TwoElemArray

deriveJSON :: Options -> Name -> Q [Dec]
deriveJSON _ _ = return []

-- Bug.hs
{-# LANGUAGE TemplateHaskell #-}
module Bug where

import Bug2
import Language.Haskell.TH

data Bad = Bad { _bad :: String } deriving (Eq, Ord, Show)

$(deriveJSON defaultOptions{} ''Bad)

$ /opt/ghc/head/bin/ghc Bug.hs
[1 of 2] Compiling Bug2             ( Bug2.hs, Bug2.o )
[2 of 2] Compiling Bug              ( Bug.hs, Bug.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.1.20161010 for x86_64-unknown-linux):
        compiler/typecheck/TcExpr.hs:820:15-35: Irrefutable pattern failed for pattern sel_id : _


Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

changed the description

Thank you Ryan!

Well that's odd. I get

T12788.hs:11:14: error: Empty record update

which comes from RnPat.rnHsRecUpdFields. And is quite correct.

If I change the program to

$(deriveJSON defaultOptions{unwrapUnaryRecords = True} ''Bad)

then I get

T12788.hs:11:14: error:
    Variable not in scope: defaultOptions :: Options

which is also correct; defaultOptions is not in scope.

All this happens with the 8.0.2 release candidate and HEAD. I can't easily try 8.0 itself.

Can you try with the 8.0 branch and HEAD?

It really would be better if the "empty record update" didn't mask the other error, namely that defaultOptions isn't in scope. Reason is the latter is reported by the type checker so that we can give the type of the out-of-scope variable, whereas the former comes from the renamer.

Regardless, I can't make it crash. Maybe it got fixed, somehow? Can you double-check the repro case.

Ah! It turns out the error I experienced was with versions of GHC 8.0.2 and HEAD built on October 10. I experienced the same errors as you reported when using:

A GHC 8.0.2 built on October 18
A GHC HEAD built on October 23

So this GHC panic must have been fixed recently. I'll try to find the offending commit.

Ah, it turns out to be bce99086 (a.k.a. #12584 (closed)) yet again.

OK good, thank you. So we can close as fixed?

mentioned in commit ec22bacd

changed milestone to %8.0.2

I added a regression test to be safe.

Since this fix was backported to 8.0.2, it should be safe to merge that commit as well.

added 1 deleted label

closed

Merged to ghc-8.0 as b72b7c8a.

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Trac field	Value
Resolution	Unresolved → ResolvedFixed

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While using Data.Aeson.TH, "Irrefutable pattern failed for pattern sel_id : _"

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