[GHC] #13443: Typeclass resolution errors quite puzzling

[GHC]

#13443: Typeclass resolution errors quite puzzling
-------------------------------------+-------------------------------------
           Reporter:  tomjaguarpaw   |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           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 ticket is based on a post I made to haskell-cafe:
 https://mail.haskell.org/pipermail/haskell-cafe/2016-August/124622.html

 Here's a program with an odd error message (GHC 8.0.1):

 {{{
     data A a = A a deriving Eq
     data B = B

     main :: IO ()
     main = print (A B == A B)

     test/main.hs:5:15: error:
         • No instance for (Eq B) arising from a use of ‘==’
         • In the first argument of ‘print’, namely ‘(A B == A B)’
           In the expression: print (A B == A B)
           In an equation for ‘main’: main = print (A B == A B)
 }}}

 I get an error about `Eq B` even though it's `Eq A` that is manifestly
 required at the call site.  This error is odder when `A` and `B` are
 defined far away from the use of `==`.

 This is even odder:

 {{{
     data A a = A a
     data B = B

     instance Ord a => Eq (A a) where

     main :: IO ()
     main = print (A B == A B)

     test/main.hs:7:15: error:
         • No instance for (Ord B) arising from a use of ‘==’
         • In the first argument of ‘print’, namely ‘(A B == A B)’
           In the expression: print (A B == A B)
           In an equation for ‘main’: main = print (A B == A B)
 }}}

 Now not only is the type puzzling (`B` instead of `A`) but the *class* is
 puzzling (`Ord` instead of `Eq`).  This occurred to me in practice because
 `Data.Graph.Inductive.PatriciaTree.Gr` has `(Eq a, Ord b) => Eq (Gr a b)`.

 It would have been a lot more helpful to see

 {{{
     * No instance for (Ord B)
         * arising from (Eq A)
         * arising from the use of '=='
 }}}

 Does anyone agree with me that GHC should produce the full trace when it
 fails to resolve instances rather than just the proximal failure?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13443>
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