#12921: initTc: unsolved constraints -------------------------------------+------------------------------------- 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: | -------------------------------------+------------------------------------- Comment (by gnezdo): I hit this problem too. I couldn't quickly locate a prebuilt version of ghc HEAD. Reporting the reproducer here instead. Given this source: {{{ {-# LANGUAGE DataKinds #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE UndecidableInstances #-} 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 }}} In ghci I get the following error: {{{ Prelude> Log2 (2 :: GHC.TypeLits.KnownNat) ghc: panic! (the 'impossible' happened) (GHC version 8.0.2 for x86_64-unknown-linux): initTc: unsolved constraints WC {wc_insol = [W] Log2_a1P3 :: t_a1P2[tau:1] (CHoleCan: Log2) [W] Log2_a1Px :: t_a1Pw[tau:1] (CHoleCan: Log2)} }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12921#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler