I had a module with unused name txFees in it. I started compilation and in the same time edited code by renaming txFees -> _txFees.

I got the following message:

    warning: [-Wunused-matches]
        Defined but not used: ‘txFees’
       |
    40 |             } _txFees
       |               ^^^^^^
    
    <no location info>: error: 
    Failing due to -Werror.

The screenshot is in the attachment. Notice that message part is about old code (txFees), and preview is about new code (_txFees).