First of all thank you for this great check!
In this patch I propose one small cleanup and two changes in the behaviour. The first is to desugar decayed types in the arguments of functions. This way it is possible to detect override candidates when a method that has a pointer as an argument tries to override a method that has an array as an argument that is decayed to a pointer. The other change is not to take visibility into account since it is perfectly valid to override a public method with a private one and such code exists.
I have one more proposal that is not implemented in this patch yet: it is a common mistake to not to override a method because the developer forget to add the qualifiers. What about a configuration option to also report near misses when only a qualifier is missing?
What do you think?