[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 rwbarton):

 The parts of this I feel strongly about are

 * GHC should never warn about a command line that is valid. It will just
 cause too many headaches for build systems. The effects of options should
 be clearly documented and then GHC should do what the user asks without
 any second-guessing. If a combination of options really doesn't make
 sense, then it should be an error and not a warning.

 * `-ffoo -fno-foo` and `-fno-foo -ffoo` (and `-O1 -O0`, etc.) should be
 valid with the last flag taking priority, as is the case today.

 I'm not really qualified to comment on how obvious the current behavior of
 `-fno-specialise -O1` is (it's obvious to me, but I've had the dubious
 pleasure of reading through DynFlags.hs on many occasions). I also can't
 think of a use case where the rule suggested in comment:2 "process all
 `-O` flags before all `-f` flags" would prevent one from doing something
 useful. So it sounds OK to me. But no warnings please!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10560#comment:15>
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