[GHC] #15850: coercion errors don't always mention that data constructors are not in scope

[GHC]

#15850: coercion errors don't always mention that data constructors are not in
scope
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           Reporter:  gelisam        |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Suppose a library defines a `newtype` with a `nominal` type parameter:

 {{{#!hs
 {-# LANGUAGE RoleAnnotations #-}
 module Lib where

 type role LibIdentity nominal
 newtype LibIdentity a = LibIdentity a
 }}}

 (in my code, that `newtype` is `ExceptT`, whose last argument is inferred
 to be `nominal`)

 In a different module, I also define a `newtype`, and then I try to use
 `coerce` to remove some of the `newtype` wrappers. Unfortunately I forgot
 to import `LibIdentity`'s data constructor, and so the coercions fail.
 Sometimes, the error message helpfully explains that I need that import...
 but sometimes it doesn't!

 {{{#!hs
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 module Local where

 import Data.Coerce (coerce)
 import Lib (LibIdentity)

 newtype LocalIdentity a = LocalIdentity a

 -- error:
 --     • Couldn't match representation of type ‘()’
 --                                with that of ‘LibIdentity ()’
 --         arising from a use of ‘coerce’
 --       The data constructor ‘Lib.LibIdentity’
 --         of newtype ‘LibIdentity’ is not in scope
 helpfulError :: LocalIdentity (LibIdentity ()) -> LocalIdentity ()
 helpfulError = coerce

 -- error:
 --     • Couldn't match type ‘LocalIdentity ()’ with ‘()’
 --         arising from a use of ‘coerce’
 difficultError :: LibIdentity (LocalIdentity ()) -> LibIdentity ()
 difficultError = coerce
 }}}

 Expected: both uses of `coerce` remind me to import the data constructor
 of `LibIdentity`.

 Observed: only `helpfulError` reminds me, while `difficultError` doesn't.

 (in my code, the `coerce` is generated by `GeneralizedNewtypeDeriving` and
 the error message does not include the type whose data constructor needs
 to be imported, making it quite difficult to guess how to fix the error)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15850>
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