Ambiguous inferred type causes a panic

Consider the following program.

{-# LANGUAGE NoMonomorphismRestriction, PartialTypeSignatures #-}

module Main where

import Text.Parsec
import Text.Parsec.String

parser :: Parser _
parser = read <$> many digit

data Wrapper = Wrapper Int deriving Show

wrapperParser = Wrapper <$> parser

main :: IO ()
main = parseTest wrapperParser "0"

I am not sure whether it is valid or not, but it breaks the type checker as follows.

Main.hs:9:1:
    No instance for (Read w_)
    When checking that ‘parser’ has the specified type
      parser :: forall w_. Parser w_
    Probable cause: the inferred type is ambiguous

Main.hs:13:29:
    Couldn't match type ‘w_’ with ‘Int’
      ‘w_’ is untouchable
        inside the constraints ()
        bound by the inferred type of
                 wrapperParser :: ParsecT
                                    String () Data.Functor.Identity.Identity Wrapper
        at Main.hs:13:1-34ghc: panic! (the 'impossible' happened)
  (GHC version 7.10.2 for x86_64-unknown-linux):
        No skolem info: w__a2BN[sk]

Disabling PartialTypeSignatures makes no difference. Switching from parsec to megaparsec and changing many digit to some digitChar does not have an effect either. Removing NoMonomorphismRestriction makes the problem disappear, as does changing _ to Int.

The libraries used were

• array-0.5.1.0,
• base-4.8.1.0,
• binary-0.7.5.0,
• bytestring-0.10.6.0,
• containers-0.5.6.2,
• deepseq-1.4.1.1,
• ghc-prim-0.4.0.0,
• integer-gmp-1.0.0.0,
• megaparsec-4.1.1,
• mtl-2.2.1,
• parsec-3.1.9,
• text-1.2.1.3 and
• transformers-0.4.2.0.