Obviously I cannot show the code base, but when I pick a pre-existing solution from Stackoverflow or elsewhere—though it is quite rare—I do add a comment linking to the source: after all, in case of SA the discussion there might be interesting for the future maintainers of the function.

I just checked, though, and the code base I'm now working with has eight stackoverflow links. Not all are even written by me, according to quick check with git blame and git log -S..