JSONPath isn't tracked in autoderived instances

[neongreen]

Let's create a type and derive an instance for it.

With TH:

{-# LANGUAGE TemplateHaskell #-}

import Data.Aeson
import Data.Aeson.TH

data X = X {a :: (Int, Int, Int)} 
  deriving Show

deriveJSON defaultOptions ''X

Or with generics:

{-# LANGUAGE DeriveGeneric #-}

import Data.Aeson

data X = X {a :: (Int, Int, Int)}
  deriving (Show, Generic)

instance FromJSON X where
  parseJSON = genericParseJSON defaultOptions 

Then try to decode an invalid value:

> eitherDecode "{\"a\": [1, 2, true]}" :: Either String X
Left "Error in $[2]: expected Int, encountered Boolean"

I believe it should be $.a[2].

(I checked with latest Aeson from the repository, commit 2f0b989.)

[neongreen]

Just in case, here's the output of encode (to see that encoded JSON indeed has "a"):

> encode (X (1,2,3))
"{\"a\":[1,2,3]}"

[neongreen]

Relevant code in Data.Aeson.TH is here: https://github.com/bos/aeson/blob/master/Data/Aeson/TH.hs#L992. For some reason it uses a manual lookup instead of .::

    lookupField tName rec obj key =
        case H.lookup key obj of
          Nothing -> unknownFieldFail tName rec (T.unpack key)
          Just v  -> parseJSON v

I'm not sure where the code for generics is, but it's probably here: https://github.com/bos/aeson/blob/master/Data/Aeson/Types/Generic.hs#L670:

instance (Selector s, GFromJSON a) => FromRecord (S1 s a) where
    parseRecord opts (Just lab) = maybe (notFound $ unpack lab)
                      (gParseJSON opts) . H.lookup lab
    parseRecord opts Nothing    = maybe (notFound label)
                      (gParseJSON opts) . H.lookup (pack label)
        where
          label = fieldLabelModifier opts $ selName (undefined :: t s a p)

(It's the only place in Data.Aeson.Types.Generic where lookup is mentioned.)