[GHC] #15052: DeriveAnyClass instances may skip TypeError constraints

[GHC]

#15052: DeriveAnyClass instances may skip TypeError constraints
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        Reporter:  jcpetruzza        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:
                                     |  CustomTypeErrors
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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Changes (by RyanGlScott):

 * keywords:   => CustomTypeErrors


Comment:

 Ah. You're arguing something different, then. You're proposing that if you
 have a type class instance with `TypeError` in the context (such as the
 three instances in comment:1), then GHC should reject that instance
 itself, instead of rejecting //uses// of that instance.

 This would be a pretty big breaking change. There is a lot of code out
 there which relies on the ability to use `TypeError` constraints in
 instance contexts. (For examples see the
 [http://git.haskell.org/ghc.git/blob/5d76846405240c051b00cddcda9d8d02c880968e:/testsuite/tests/deriving/should_compile/T14339.hs#l16
 T14339 test case].) I think if you want to see this behavior be enshrined
 into GHC, you should first submit a proposal to the [https://github.com
 /ghc-proposals/ghc-proposals GHC proposals repo], and argue why this
 behavior is more desirable over the status quo.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15052#comment:6>
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