[GHC] #15290: QuantifiedConstraints: panic "addTcEvBind NoEvBindsVar"

[GHC]

#15290: QuantifiedConstraints: panic "addTcEvBind NoEvBindsVar"
-------------------------------------+-------------------------------------
           Reporter:  goldfire       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
           Keywords:                 |  Operating System:  Unknown/Multiple
  QuantifiedConstraints              |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I wanted to see if we're ready to put `join` into `Monad`. So I typed this
 in:

 {{{#!hs
 {-# 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
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15290>
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