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

[GHC]

#14170: 8.2.1 regression: GHC fails to simplify `natVal`
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        Reporter:  vagarenko         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.2
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 I was able to track this down to commit
 1fcede43d2b30f33b7505e25eb6b1f321be0407f (`Introduce GHC.TypeNats module,
 change KnownNat evidence to be Natural`), which hints at the problem. In
 that commit, we switched the internal representation of `Nat`s from
 `Integer`s to `Natural`s (from `Numeric.Natural`). For whatever reason,
 however, `Natural` values don't seem to simplify as well as `Integers`, as
 evidenced by this simpler program:

 {{{#!hs
 module Bug where

 import Numeric.Natural

 foo :: Natural
 foo = 0
 }}}

 which also produces essentially identical core:

 {{{#!hs
 -- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
 Bug.foo1 :: Integer
 [GblId,
  Caf=NoCafRefs,
  Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True,
          WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 100 0}]
 Bug.foo1 = 0

 -- RHS size: {terms: 39, types: 12, coercions: 0, joins: 0/0}
 foo :: Natural
 [GblId,
  Unf=Unf{Src=<vanilla>, TopLvl=True, Value=False, ConLike=False,
          WorkFree=False, Expandable=False, Guidance=IF_ARGS [] 126 60}]
 foo
   = case Bug.foo1 of {
       integer-gmp-1.0.1.0:GHC.Integer.Type.S# i#_a2bZ ->
         case GHC.Prim.tagToEnum# @ Bool (GHC.Prim.>=# i#_a2bZ 0#) of {
           False -> GHC.Natural.underflowError @ Natural;
           True -> GHC.Natural.NatS# (GHC.Prim.int2Word# i#_a2bZ)
         };
       integer-gmp-1.0.1.0:GHC.Integer.Type.Jp# dt_a2c9 ->
         case GHC.Prim.uncheckedIShiftRL#
                (GHC.Prim.sizeofByteArray# dt_a2c9) 3#
         of {
           __DEFAULT ->
             case GHC.Prim.sizeofByteArray# dt_a2c9 of {
               __DEFAULT -> GHC.Natural.NatJ# dt_a2c9;
               0# -> GHC.Natural.underflowError @ Natural
             };
           1# ->
             case GHC.Prim.indexWordArray# dt_a2c9 0# of wild2_a2cd
             { __DEFAULT ->
             GHC.Natural.NatS# wild2_a2cd
             }
         };
       integer-gmp-1.0.1.0:GHC.Integer.Type.Jn# ipv_a2cg ->
         GHC.Natural.underflowError @ Natural
     }
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14170#comment:1>
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