[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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Comment (by jcpetruzza):

 Ok, I think I understand your point: what we see follows from the
 documented behaviour for `DerivingAnyClass` so there is really no bug
 here.

 Tbh, I hadn't noticed until right now that `deriving` would propagate
 constraints coming from the default signatures! The initial example in the
 docs says:

 > {{{#!haskell
 >  data Foo = Foo deriving (Show, SPretty)
 > }}}
 >
 >  The above code is equivalent to:
 >
 > {{{#!haskell
 > data Foo = Foo deriving Show
 > instance SPretty Foo
 > }}}

 and somehow the idea that `deriving` would declare the instance over an
 empty context got stuck in my head for a long time. Hence my insistance in
 expecting the results to be the same! (maybe `Foo a` would make for a
 better example there?)

 In any case, I agree that `deriving` is doing the right thing here. It is
 a bit unfortunate since I'm not sure I  want to use `TypeError` as it is
 for generic default methods then, but on the other hand, without it error
 messages can be a screenful!

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15052#comment:13>
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