Yeah, I think gcc has a std c++20 mode as well as a with extensions mode. And adding `-pedantic` helps because it forces extensions (unless specified in the std type) and non-conformant code to be rejected.
And that clang likely just didn't have full c++20 support. Which tbf I actually don't think any clang has full c++20 support currently as even bleeding edge clang still is missing a few things in the lang and library departments.