That's something that will unfortunately never work with ctags and will have to wait for LSP (aka the post-ctags time consumer if I or someone else is brave enough).
Yeah, that would make Geany into a "full fat" IDE, not sure we want that. But inference is why I moved my C++ work to one. I guess (presuming "somebody" offers to do it) that is a decision that will be needed some day as more languages use inference, but not now.
The question is what we want - whether icons corresponding to groups or icons corresponding to tag types like in the symbol tree - see https://github.com/geany/geany/pull/3176. I suggest to try the sorting only first and see how it behaves.
I thought I saw something about icons recently!!! Yeah, its not this PR, sorting for a start.
For now I'll just keep it as it is - it didn't work before, it won't work now - and can be improved later.
Agreed, as I said "not this PR".