QuantifiedConstraints: panic "addTcEvBind NoEvBindsVar"

I wanted to see if we're ready to put join into Monad. So I typed this in:

{-# LANGUAGE QuantifiedConstraints, StandaloneDeriving, GeneralizedNewtypeDeriving #-}

module Bug where

import Prelude hiding ( Monad(..) )
import Data.Coerce ( Coercible )

class Monad m where
  (>>=) :: m a -> (a -> m b) -> m b
  join  :: m (m a) -> m a

newtype StateT s m a = StateT { runStateT :: s -> m (s, a) }

instance Monad m => Monad (StateT s m) where
  ma >>= fmb = StateT $ \s -> runStateT ma s >>= \(s1, a) -> runStateT (fmb a) s1
  join ssa = StateT $ \s -> runStateT ssa s >>= \(s, sa) -> runStateT sa s

newtype IntStateT m a = IntStateT { runIntStateT :: StateT Int m a }

deriving instance (Monad m, forall p q. Coercible p q => Coercible (m p) (m q)) => Monad (IntStateT m)

This looks like it should be accepted. But I get

ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.5.20180617 for x86_64-apple-darwin):
	addTcEvBind NoEvBindsVar
  [G] df_a67k
    = \ (@ p_a62C) (@ q_a62D) (v_B1 :: Coercible p_a62C q_a62D) ->
        coercible_sel
          @ *
          @ (m_a64Z[ssk:1] p_a62C)
          @ (m_a64Z[ssk:1] q_a62D)
          (df_a651 @ p_a62C @ q_a62D v_B1)
  a67c
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1164:37 in ghc:Outputable
        pprPanic, called at compiler/typecheck/TcRnMonad.hs:1404:5 in ghc:TcRnMonad

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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