https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92057
--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> --- Because the narrowing conversion happens inside the Double(int) constructor. The constrain on the variant(T&&) constructor only checks if a Double can be constructed from an int without narrowing. Which it can. What happens inside the constructor after that is unrelated. That's how C++ works.
