https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115720
--- Comment #1 from cqwrteur <unlvsur at live dot com> --- https://www.reddit.com/r/cpp/comments/1dqhlh2/c_modules_discussion_ask_questions_and_share_your/ import std; #include <vector> // or any other standard library header Looks like defects in the C++ standard that does not support it. I suggest the solution is to allow the "inline" class to discord duplicated classes or templates across different modules.