Unused binding warning should not apply to newtype constructors

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.

Trac metadata

Trac field	Value
Version	8.6.1-beta1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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