Inconsistent unused variable warnings

Summary

1. Function-argument bound, let bound and where bound variables are treated inconsistently when it comes to unused variable warnings.
2. Variables appearing on the RHS of a strict binding should be considered "used".

Steps to reproduce

% cat test1.hs && /opt/ghc/bin/ghc-8.10.2 test1.hs
{-# LANGUAGE BangPatterns #-}

{-# OPTIONS_GHC -Wall #-}

module Foo where

foo :: a -> ()
foo a = let _ = undefined a b c
            b = ()
        in ()
  where c = ()

bar :: a -> ()
bar a = let !_ = undefined a b c
            b = ()
        in ()
  where c = ()
[1 of 1] Compiling Foo              ( test1.hs, test1.o )

test1.hs:9:13: warning: [-Wunused-local-binds]
    Defined but not used: ‘b’
  |
9 |             b = ()
  |             ^

test1.hs:15:13: warning: [-Wunused-local-binds]
    Defined but not used: ‘b’
   |
15 |             b = ()
   |             ^

Expected behavior

1. For each function there should either be a warning for all of a, b and c or for none of them.
2. I would also expect a strict binding to "use" all the variables on its RHS, even if it binds no variables.

Environment

8.10.2 from HVR's Ubuntu PPA