Duplicate role annotations [GHC-97170]

This error occurs if more than one role annotation occurs for a given datatype. Note that the role annotations need not be conflicting for this error to arise. Also note that the role annotations can occur in different locations from the datatype.

Examples

Double role annotation on datatype

Error Message

DoubleRoleAnnotationOnData.hs:6:1: error: [GHC-97170]
    Duplicate role annotations for ‘Foo’:
      type role Foo nominal
          -- written at DoubleRoleAnnotationOnData.hs:5:1-21
      type role Foo representational
          -- written at DoubleRoleAnnotationOnData.hs:6:1-30
  |
6 | type role Foo representational
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

DoubleRoleAnnotationOnData.hs

Before

{-# LANGUAGE RoleAnnotations #-}
module DoubleRoleAnnotationOnData where

data Foo a = MkFoo a
type role Foo nominal
type role Foo representational

After

{-# LANGUAGE RoleAnnotations #-}
module DoubleRoleAnnotationOnData where

data Foo a = MkFoo a
type role Foo nominal