[[nodiscard]] and KDE coding policy

I'm seeing more and more merge requests using [[nodiscard]] and now
review comments insisting on others adding [[nodiscard]] to new code.

I've seen everyone has their own set of rules:
 - some don't use it
 - some add it when the return value is important (like opening a file
having an error)
 or when the return value has a RAII pattern or requires the caller to
take over memory ownership
 - some add it to every getter

I don't like inconsistency and I don't like repeated discussions on
merge requests.

I would like to as a group decide on something and add it to
https://community.kde.org/Policies/Library_Code_Policy at least for
new code with a definitive rule that everyone has to follow.

Whether we change existing code is also up for discussion it's a
source-incompatible change even though it's binary compatible. We
normally avoid source compatible changes, but it shouldn't make too
much difference unless we're exposing existing mistakes - which is
something we want to do.

Any thoughts?

David