[GHC] #14170: 8.2.1 regression: GHC fails to simplify `natVal`

[GHC]

#14170: 8.2.1 regression: GHC fails to simplify `natVal`
-------------------------------------+-------------------------------------
           Reporter:  vagarenko      |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.2.2
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 When GHC 8.2.1 compiles this code with `-O`:
 {{{#!hs
 {-# LANGUAGE TypeApplications      #-}
 {-# LANGUAGE TypeInType            #-}

 module NatVal where

 import Data.Proxy
 import GHC.TypeLits

 foo = natVal $ Proxy @0
 }}}

 it produces the following Core:

 {{{#!hs
 -- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
 NatVal.foo1 :: Integer
 NatVal.foo1 = 0

 -- RHS size: {terms: 41, types: 18, coercions: 0, joins: 0/0}
 foo :: Integer
 foo
   = case NatVal.foo1 of wild_a1iV {
       integer-gmp-1.0.1.0:GHC.Integer.Type.S# i#_a2ke ->
         case GHC.Prim.tagToEnum# @ Bool (GHC.Prim.>=# i#_a2ke 0#) of {
           False -> case GHC.Natural.underflowError of wild2_00 { };
           True ->
             integer-gmp-1.0.1.0:GHC.Integer.Type.wordToInteger
               (GHC.Prim.int2Word# i#_a2ke)
         };
       integer-gmp-1.0.1.0:GHC.Integer.Type.Jp# dt_a2km ->
         case GHC.Prim.uncheckedIShiftRL#
                (GHC.Prim.sizeofByteArray# dt_a2km) 3#
         of {
           __DEFAULT ->
             case GHC.Prim.sizeofByteArray# dt_a2km of {
               __DEFAULT -> wild_a1iV;
               0# -> case GHC.Natural.underflowError of wild4_00 { }
             };
           1# ->
             case GHC.Prim.indexWordArray# dt_a2km 0# of wild2_a2kq
             { __DEFAULT ->
             integer-gmp-1.0.1.0:GHC.Integer.Type.wordToInteger wild2_a2kq
             }
         };
       integer-gmp-1.0.1.0:GHC.Integer.Type.Jn# ipv_a2kt ->
         case GHC.Natural.underflowError of wild1_00 { }
     }
 }}}

 while GHC-8.0.1 does the right thing:
 {{{#!hs
 -- RHS size: {terms: 1, types: 0, coercions: 0}
 foo :: Integer
 foo = 0
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14170>
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