Spurious "unused constructor" warning with Coercible (#10347) · Issues · Glasgow Haskell Compiler / GHC

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

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.

<details><summary>Trac metadata</summary>

| 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           |              |

</details>