[GHC] #15569: Constant folding optimises 1 into 3

[GHC]

#15569: Constant folding optimises 1 into 3
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        Reporter:  snowleopard       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.6.1-beta1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9136             |  Differential Rev(s):  Phab:D5109
       Wiki Page:                    |
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Comment (by monoidal):

 I picked all rules from D2858, including the wrong and the correct version
 of `(L y :-:   v) :-: (L x :-: w)` and used sympy to compute the
 difference between LHS and RHS symbolically. Result: all differences
 simplify to 0, except that one rule that was found to be wrong. Code:
 https://gist.github.com/monoidal/3d5565b986a013639389fc57081d2781

 All of the rules use addition, subtraction and multiplication only
 (matters would be different if there was division). This makes me
 confident that the rules are algebraically correct. It doesn't mean the
 code is completely correct, but any remaining errors will be of different
 nature. If someone is willing to do further review, I would suggest things
 like: double-checking the code surrounding the rules, checking if integer
 overflow can occur, does this apply only to Int/Integer or other types
 etc.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15569#comment:20>
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