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

[GHC]

#10117: Change the scheme for reporting redundant imports
-------------------------------------+-------------------------------------
        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 ezyang):

 For backwards compatibility, new versions of a module may add new exports
 but will not remove exports. The problem with the existing unused imports
 warning is that warnings can arise even when an import is upgraded in a
 backwards-compatible way (a minor version bump). For example:

 {{{
 import Prelude
 import Control.Applicative ((<*), pure)
 }}}

 Currently, the `pure` export from `Control.Applicative` is not redundant
 if `Prelude` does not export `pure`, but is redundant if it does.  But the
 addition of `pure` to `Prelude`'s exports is OK for a minor version bump,
 which is what makes this warning "annoying".  To rule these out, here is
 our principle:

 * **Backwards-compatible upgrades do not add warnings:** if a module is
 warning free, if we upgrade the minor version (as per the PVP) of any of
 its imports, the module should continue to be warning free.

 Here are warnings which are OK:

 * `import A` is not used at all.  (For a module to be warning free, it
 must use some item from this import; but no matter how we upgrade other
 imports, that item will continue to be used.)

 * `import A (x)` is not used at all.  (Similar.)

 * An entity is imported twice explicitly, e.g., `import A (x); import B
 (x)`.  (This is always redundant (assuming no references to `A.x` or
 `B.x`), it doesn't matter what the exports of `A` and `B` are.)

 * Two imports are textually redundant, e.g., `import A; import A (x)`.
 (Once again, always redundant no matter what the exports.)

 Here are some warnings which are NOT ok:

 * `import A; import B (x)`, where `A` brings `x` into scope. An old
 version of `A` may not export `x` which would make this import-list
 warning free.

 * `import A; import B`, where `A` and `B` bring `x` into scope. Similar.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10117#comment:6>
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