mkCastTy sometimes drops insoluble (Type ~ Constraint) coercions

EDIT: See ticket:15918#comment:164033 for the real cause and ticket:15918#comment:169361 for the current plan.

See also:

• TyCoRep Note [Respecting definitional equality] and its supporting Note [Non-trivial definitional equality]
• #11715 (closed) Constraint vs *
• #15799 (closed)
• #13650

TL;DR: there's a bug in here, that we don't know how to solve. But it's not biting us, and once we solve #11715 (closed) that may point the way.

---

Minimized from https://gist.github.com/Icelandjack/683bd4b79027695ffc31632645c9d58b, I don't expect Build [] to kind check but ti shouldn't crash.

{-# Language PolyKinds             #-}
{-# Language TypeFamilies          #-}
{-# Language ConstraintKinds       #-}
{-# Language FlexibleContexts      #-}
{-# Language QuantifiedConstraints #-}
{-# Language UndecidableInstances  #-}

import Data.Kind

class Rev f where
  rev :: f a

instance (forall xx. cls xx => Rev xx) => Rev (Build cls) where
  rev = undefined

data Build :: ((k -> Type) -> Constraint) -> (k -> Type)

uu = rev :: Build [] a

gives a panic

$ ./ghc-stage2 --interactive -ignore-dot-ghci ~/hs/711_bug.hs
GHCi, version 8.7.20181029: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( ~/hs/711_bug.hs, interpreted )
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.7.20181029 for x86_64-unknown-linux):
        ASSERT failed!
  irred_a1zW :: [(xx_a1zV[sk:3] |> Sym {co_a1zu})]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable
        pprPanic, called at compiler/utils/Outputable.hs:1219:5 in ghc:Outputable
        assertPprPanic, called at compiler/typecheck/TcType.hs:1826:53 in ghc:TcType

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

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