[GHC] #15459: Wredundant-constraints does not work when constraint synonym is used

[GHC]

#15459: Wredundant-constraints does not work when constraint synonym is used
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        Reporter:  flip101           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      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 flip101):

 Simon, i don't know about type families. I tried reading about them at
 https://wiki.haskell.org/GHC/Type_families but even so i don't feel i have
 enough experience with them to give a useful response to your question.
 It's also more confusing for me because in your example foo doesn't have
 an implementation. With `withoutWarning = sort` it was clear to me that
 sort requires Ord but not Show.

 I will make a guess here and say "yes i expect a warning for that". When
 the instance `F [a]` is used in used in the function `foo` it can be
 deduced that this is the is the active instance which is used in the
 function foo. If foo is implemented as `foo = sort` then (`Show a`) is
 also redundant in the instance of the .. constraint type family ?

 My guess can be non-sense .. please refer to the first paragraph :P

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15459#comment:2>
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