[GHC] #15618: Unused binding warning should not apply to newtype constructors

[GHC]

#15618: Unused binding warning should not apply to newtype constructors
-------------------------------------+-------------------------------------
           Reporter:  dfeuer         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.6.1-beta1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  error/warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 module Foo (A, foo) where
 import Data.Type.Coercion

 newtype A = A Int

 foo :: Coercion A Int
 foo = Coercion
 }}}

 produces a warning:

 {{{
 NTCon.hs:4:13: warning: [-Wunused-top-binds]
     Defined but not used: data constructor ‘A’
   |
 4 | newtype A = A Int
   |             ^^^^^
 }}}

 This is rather silly!

 1. It's ''impossible'' to define a newtype without giving it a data
 constructor.

 2. It's ''possible'' to use a newtype without using its data constructor.

 Therefore, I believe the only reasonable thing to do is suppress the
 unused binding warning for newtype data constructors. If the newtype
 itself isn't used, that will be caught anyway.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15618>
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