Spurious "unused constructor" warning with Coercible
When I say
{-# LANGUAGE RoleAnnotations #-}
module Bug (N, twiddle) where
import Data.Coerce
newtype N a = MkN Int
type role N representational
twiddle :: N a -> N b
twiddle = coerceI get
Bug.hs:7:15: warning: Defined but not used: data constructor ‘MkN’But the constructor is used, in order to do the coercion. The role annotation is to force GHC to use the constructor to do the coercion -- otherwise, it might not have.
This bug exists in 7.10 and in HEAD.
I will fix.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 7.11 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |