Created attachment 153724 [details] The relevent window In the project configuration, under Language Support, you can choose the C++ profile. The support for c++20 is written as c++2a. Starting from GCC 10, the correct command-line parameter is c++20. See: https://gcc.gnu.org/projects/cxx-status.html#cxx20
Fixed in https://invent.kde.org/kdevelop/kdevelop/-/commit/56b53ac50fd7db273dddf863650e37257791c1dc