[GHC] #16362: Deriving a class via an instance that has a TypeError constraint using standalone deriving fails during compilation.

[GHC]

#16362: Deriving a class via an instance that has a TypeError constraint using
standalone deriving fails during compilation.
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           Reporter:  j9794          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This bug occurs if I define an instance for a typeclass adding a TypeError
 constraint on the instance, and then I try to derive that instance using
 Deriving Via in combination with StandaloneDeriving.
 This is the shortest example I was able to come up with:

 {{{
 {-# Language DataKinds, UndecidableInstances, StandaloneDeriving,
 DerivingVia #-}

 import GHC.TypeLits

 newtype NotNum a = NotNum a

 instance (TypeError (Text "Not a num")) => Num (NotNum a) where

 data Foo = Foo
 deriving via (NotNum Foo) instance Num Foo
 }}}

 In this case, it'll fail in compilation with 'Not a Num'.

 This only seems to happen when combining deriving via with standalone
 deriving, because if I derive the class like:

 {{{
 data Foo = Foo deriving Num via (NotNum Foo)
 }}}

 It works as expected (doesn't fail with my custom error until I actually
 try to use a Foo where I should use a Num)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16362>
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