[GHC] #10117: Change the scheme for reporting redundant imports

[GHC]

#10117: Change the scheme for reporting redundant imports
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        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.8.4
      Resolution:                    |             Keywords:  deprecate
                                     |  warning
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 You state a principle; that's good.

 Can you refine it a bit?

 * Rather than referring indirectly to the PVP, in addition say exactly
 what changes should not add warnings.  I think the specific issue is this:
 if a module M compiles without warnings, and one of the modules it imports
 adds an extra export, then M should still compile without warnings.

 * Can you give a refined form of the old principle? Something like: an
 import item should be reported as redundant if it can be deleted without
 changing the meaning of the program, EXCEPT that a specific `import B(x)`
 is not reported as redundant if the only other way `x` is in scope is via
 whole-module imports like `import A` or `import A hiding...`.

   I think that's not quite right, but it'd be a helpful second step.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10117#comment:7>
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