Bug found with ghc-option=-Wunused-matches using interfaces

Questions Features & Bugs
1

--------clean project with only this module -----

module Foo where

data V_Foo =  V_Foo with 
    owner : Party
  deriving (Eq, Ord, Show)

interface I_Foo
  where
    viewtype V_Foo

    nonconsuming choice Inc : Int
      with
        x        : Int
      controller (view this).owner
      do
        return $ x + 1

----- build options-----

sdk-version: 2.6.4
name: daml-intro-to-interfaces
build-options: 
- target=1.15
- ghc-option=-Wunused-matches

source: daml
version: 0.0.1
dependencies:
  - daml-prim
  - daml-stdlib
  - daml-script
sandbox-options:
  - wall-clock-time

: I get the following error : : warning: Defined but not used: ‘arg’