[GHC] #11066: Inacessible branch should be warning - otherwise breaks type soundness?

[GHC]

#11066: Inacessible branch should be warning - otherwise breaks type soundness?
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        Reporter:  rrnewton          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #8128, #8740      |  Differential Rev(s):  Phab:D1454
       Wiki Page:                    |
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Comment (by simonpj):

 It certainly sounds wrong for GHC to reject a branch as inaccessible (i.e.
 definitely cannot be executed) when it actually ''can'' be executed.  Can
 you make an example that doesn't depend on relatively sophisticated
 package?

 I tried
 {{{
 {-# LANGUAGE FlexibleContexts, TypeFamilies, TypeApplications #-}

 module T11066 where

 import Data.Typeable

 type family F a

 foo:: Typeable (F Bool) => ()
 foo = case eqT @Int @(F Bool) of
            Just Refl -> ()
            _         -> ()
 }}}
 thinking that GHC might claim that `F Bool ~ Int` is inaccesible; but it's
 accepted just fine.

 You do not say which version of the compiler you are using.

 A milder version of the problem is if the branch really is inaccessible,
 but you want to accept it anyway for some reason -- e.g. it's produced by
 `deriving`.  But that is not what you are bothered about here.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11066#comment:21>
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