[GHC] #11693: Add `When` type family

[GHC]

#11693: Add `When` type family
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        Reporter:  Iceland_jack      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Core Libraries    |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ekmett):

 This strikes me as the kind of thing that belongs out in a user library
 like `constraints` or `subhask` as it requires a fair bit of machinery to
 use well, and suffers the sort of type-level "boolean blindness" that
 indicates it'd often need to be reimplemented subtly differently.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11693#comment:1>
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