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 = coerce
I 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 |