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

#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
      Resolution:                    |             Keywords:  TypeErrors
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * keywords:   => TypeErrors


Comment:

 `data Foo = Foo deriving Num via (NotNum Foo)` is generating a slightly
 different instance than you think it is:

 {{{
 $ /opt/ghc/8.6.3/bin/ghci Bug.hs -ddump-deriv
 GHCi, version 8.6.3: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Main             ( Bug.hs, interpreted )

 ==================== Derived instances ====================
 Derived class instances:
   instance (TypeError ...) => GHC.Num.Num Main.Foo where
 }}}

 GHC treats `TypeError` constraints as insoluble, so they're inferred to be
 a part of the instance context. A standalone `deriving via (NotNum Foo)
 instance Num Foo` declaration, on the other hand, asserts that there is no
 instance context, so GHC tries to solve for the residual `TypeError`
 constraint that is inferred during typechecking. This leads to the
 compile-time exception you see in the original program.

 Note that if you write this:

 {{{#!hs
 deriving via (NotNum Foo) instance TypeError (Text "Not a num") => Num Foo
 }}}

 Or even this:

 {{{#!hs
 {-# LANGUAGE PartialTypeSignatures #-}

 deriving via (NotNum Foo) instance _ => Num Foo
 }}}

 Then you won't get an exception during compile-time.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16362#comment:2>