"see" and "copy" are two different things. It's fine to look at StackOverflow to understand the solution to a problem. It's not fine to copy and paste from StackOverflow and ignore its license or attribution.
Content on StackOverflow is under CC-by-sa, version depends on the date it was submitted: https://stackoverflow.com/help/licensing . (It's really unfortunate that they didn't pick license compatible with code; at one point they started to move to the MIT license for code, but then didn't follow through on it.)
That's a fair distinction. For the specific case of CC-by-sa 4.0, it's possible to convert to GPLv3. That doesn't help with prior versions of CC-by-sa.
So, for the specific case of material contributed to StackOverflow on or after 2018-05-02, it's possible to use it under GPLv3 (including appropriate attribution), so any project compatible with GPLv3 can copy it with attribution. Any material before that point is not safe to copy.
It does help with prior versions, since you can use CC BY-SA 3.0 material under CC BY-SA 4.0, which is GPLv3-compatible. (See https://meta.stackexchange.com/a/337742/308065.) It doesn't necessarily help with future versions.
I stand corrected, thank you! I forgot that CC-by-sa included an automatic upgrade clause, unlike many other licenses that require an explicit "or any later version" to do that.
Content on StackOverflow is under CC-by-sa, version depends on the date it was submitted: https://stackoverflow.com/help/licensing . (It's really unfortunate that they didn't pick license compatible with code; at one point they started to move to the MIT license for code, but then didn't follow through on it.)