[GHC] #12921: initTc: unsolved constraints

[GHC]

#12921: initTc: unsolved constraints
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        Reporter:  jlp               |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:  fixed             |             Keywords:
Operating System:  MacOS X           |         Architecture:  x86_64
                                     |  (amd64)
 Type of failure:  Compile-time      |            Test Case:
  crash or panic                     |  typecheck/should_fail/T12921
      Blocked By:                    |             Blocking:
 Related Tickets:  #13106            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by gnezdo):

 I located a binary and after fixing up the code it's working. Thanks!

 {{{
 {-# LANGUAGE AllowAmbiguousTypes #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE DefaultSignatures #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE UndecidableInstances #-}

 import Data.Proxy
 import GHC.TypeLits

 -- | The floor of log_2(n).
 type family Log2 n where
       Log2 1 = 0
       Log2 n = Log2 (Div2 n) + 1

 -- | The floor of n/2.
 type family Div2 n where
     Div2 0 = 0
     Div2 1 = 0
     Div2 n = Div2 (n - 2) + 1

 valueOf :: forall (n :: Nat) . (KnownNat n) => Integer
 valueOf = natVal (Proxy :: Proxy n)
 }}}


 {{{
 $ ./ghc-install/bin/ghci a.hs
 GHCi, version 8.2.0.20170704: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( a.hs, interpreted )
 Ok, 1 module loaded.
 *Main> :set -XDataKinds
 *Main> :set -XTypeApplications
 *Main> valueOf @(Log2 3)
 1

 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12921#comment:12>
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