Useful for function pointers, as for those no compiler warning fires.
This seems like a useful checker!
Don't forget to add the license.
I don't think this is necessary.
But if you do remove it you'll need to make sure to not warn when the function decl is a non-prototype declaration.
Can you document why skipping implicit declarations is the right thing to do? I'm not sure why this is necessary.
It would be awesome to mention how many arguments are expected and also add a path note indicating where the function came from.
Don't forget to add a test for the variadics case.