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

[GHC]

#15290: QuantifiedConstraints: panic "addTcEvBind NoEvBindsVar"
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 Simpler test case:

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

 module Bug where

 import Data.Coerce

 class C m where
   join :: m (m a) -> m a

 newtype T m a = MkT (m a)

 deriving instance (C m, forall p q. Coercible p q => Coercible (m p) (m
 q)) => C (T m)
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15290#comment:1>
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