https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95029
--- Comment #2 from Maximilian Matthe <maxi.matthe at googlemail dot com> --- Unfortunately, this seems to be a hard-to-spot case of the most vexing parse (just tried with clang, whose warning pointed me into that direction). https://www.fluentcpp.com/2018/01/30/most-vexing-parse/