[GHC] #4426: Simplify the rules for implicit quantification

[GHC]

#4426: Simplify the rules for implicit quantification
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        Reporter:  simonpj           |            Owner:
            Type:  feature request   |           Status:  patch
        Priority:  low               |        Milestone:  7.6.2
       Component:  Compiler          |          Version:  6.12.3
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:  #7880
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Changes (by monoidal):

 * status:  new => patch


Comment:

 Patch attached.

 It adds a new warning `-fwarn-context-quantification` that reports when a
 variable is quantified only because it under a "=>" arrow. Later, as the
 proposal says, this warning will be made an out-of-scope error.

 Normally, `HsForAll` is either `HsForAll Implicit` or `HsForAll Explicit`,
 and a context `T => R` inserts `HsForAll Implicit`. This patch adds a new
 value, `HsForAll Qualified`, which is completely equivalent to `HsForAll
 Implicit` save for the warning message. Later, its behavior will be
 changed and the renamer will not do implicit quantification for variables
 under `HsForAll Quantified`.

 I made `-Wall` not imply this flag, in (doubtful) case there is still time
 to merge this to GHC 7.8. In this case, there is full backwards
 compatibility: no change in behavior except if the flag is explicitly
 supplied. However, the API is slightly changed and I had to make a tiny
 change to Haddock. If it's too late, in GHC 7.10 I'd like `-Wall` to
 enable `-fwarn-context-quantification`, so I'll amend the patch.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4426#comment:9>
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