Missing constant folding for Numeric.Natural

The following Haskell code

module Zero where

import Numeric.Natural

zero :: Natural
zero = 0

generates this Core:

-- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
zero1
zero1 = 0

-- RHS size: {terms: 39, types: 12, coercions: 0, joins: 0/0}
zero
zero
  = case zero1 of {
      S# i#_a2c6 ->
        case tagToEnum# (>=# i#_a2c6 0#) of {
          False -> underflowError;
          True -> NatS# (int2Word# i#_a2c6)
        };
      Jp# dt_a2cg ->
        case uncheckedIShiftRL# (sizeofByteArray# dt_a2cg) 3# of {
          __DEFAULT ->
            case sizeofByteArray# dt_a2cg of {
              __DEFAULT -> NatJ# dt_a2cg;
              0# -> underflowError
            };
          1# ->
            case indexWordArray# dt_a2cg 0# of wild2_a2ck { __DEFAULT ->
            NatS# wild2_a2ck
            }
        };
      Jn# ipv_a2cn -> underflowError
    }

It should instead generate this:

zero = NatS# 0##

This kind of code is generated whenever one uses GHC.TypeLits.natVal, even at a constant Nat.

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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