[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:  Bodigrim
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.3
       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:                    |
-------------------------------------+-------------------------------------

Comment (by phadej):

 Yes, IMO there is enough motivation to have evidence as `Natural`. We are
 talking about type level Nats, not Ints.

 The fact that term value `Natural`s are second-grade citizens in GHC atm
 is unfortunate.
 If we are able to constant fold other numeric types, we should be able to
 do that for `Natural` too.

 See https://phabricator.haskell.org/D3024 discussion.

 I'm in great favour of adding `Natural` literal into core. It was
 untrivial and I didn't pursue that back then, but again IMO it's the right
 thing to do.

 Pushing even further, I'd propose to introduce `{-# LANGUAGE
 NaturalLiterals #-}`, so `2` is desugared to `2 :: FromNatural a => a`
 where `class fromNatural :: FromNatural a where Natural -> a` (use `Num`
 or better `FromInteger` if `NegativeLiterals` is enabled and literal is in
 fact negative). Putting that behind the language extension doesn't break
 old code (which needs `Num a => a`).

 Side-question: how constant folding works when cross-compiling, when `Int`
 is of different bit-widths on host and target?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14170#comment:11>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler