[GHC] #10560: -f and -O options interact in non-obvious, order dependent ways

[GHC]

#10560: -f and -O options interact in non-obvious, order dependent ways
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        Reporter:  bgamari           |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by bgamari):

 I have opened Phab:D1008 to document the current behavior.

 nomeata, I find the fact that the `-f` flags are order dependent to be
 perfectly acceptable. Moreover, I don't know how else one would handle
 this without losing expressive power.

 I think the real issue is the fact that `-O` and `-f` can silently
 conflict and one has know way of knowing this other than looking at
 `optLevelFlags` in the source. This tripped up both me and George in
 #10491. I think gidyn's suggestion of warning is a nice solution.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10560#comment:5>
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