[GHC] #14237: -Wredundant-constraints incorrectly treats all type equality constraints as redundant

[GHC]

#14237: -Wredundant-constraints incorrectly treats all type equality constraints as
redundant
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        Reporter:  lexi.lambda       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12700            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by lexi.lambda):

 I disagree that this is “very similar” to #12700. This is a **regression**
 from GHC 8.0, which did not have this problem. The type equality in the
 example given in #12700 is arguably redundant, since the program would
 compile without it, but the examples I’ve given in this ticket will fail
 to compile if the equalities are removed.

 When compiling these with `-Wredundant-constraints` in GHC 8.0.2, no
 warning is produced. In GHC 8.2.1, a warning ''is'' produced. I’m not sure
 why this happens, but it seems like a real step backwards, and it’s much
 more severe than #12700.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14237#comment:4>
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