[ 2020-04-16, Jonathan adds that [re.regex.locale] requires the traits type to be default-initialized, despite no guarantee that the traits type is default constructible. ]

The requirements on the traits class in [re.req] do not say whether a regular expression traits class is required to be DefaultConstructible, CopyConstructible, CopyAssignable etc.

The std::regex_traits class appears to be all of the above, but can basic_regex assume that for user-defined traits classes?

Should the following statements all leave u in equivalent states?

X u{v};
X u; u = v;
X u; u.imbue(v.getloc();

Whether they are equivalent has implications for basic_regex copy construction and assignment.