[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 RyanGlScott):

 gnezdo, I can confirm that your code no longer panics with GHC 8.2:

 {{{
 $ /opt/ghc/8.2.1/bin/ghci Bug.hs
 GHCi, version 8.2.0.20170704: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Main             ( Bug.hs, interpreted )
 Ok, 1 module loaded.
 λ> Log2 (2 :: GHC.TypeLits.KnownNat)

 <interactive>:1:1: error:
     Data constructor not in scope: Log2 :: t0 -> t1

 <interactive>:1:12: error:
     • Expecting one more argument to ‘KnownNat’
       Expected a type, but ‘KnownNat’ has kind ‘Nat -> Constraint’
     • In an expression type signature: KnownNat
       In the first argument of ‘Log2’, namely ‘(2 :: KnownNat)’
       In the expression: Log2 (2 :: KnownNat)
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12921#comment:11>
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