https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105995
Bug ID: 105995 Summary: QoI: constexpr basic_string variable must use all of its SSO buffer Product: gcc Version: 12.1.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: libstdc++ Assignee: unassigned at gcc dot gnu.org Reporter: de34 at live dot cn Target Milestone: --- The current implementation in libstdc++ only supports constexpr std::basic_string variable of static storage duration which uses all of its SSO buffer: #include <string> constexpr std::string str15{"0123456789abcde"}; // constexpr std::string str14{"0123456789abcd"}; // error: should we support this? // constexpr std::string str16{"0123456789abcdef"}; // error: unsupportable constexpr std::u16string u16str7{u"0123456"}; constexpr std::u32string u32str3{U"012"}; int main() { // constexpr std::string strdyn{"0123456789abcde"}; // error: unsupportable } Compiler Explorer links: https://gcc.godbolt.org/z/cvbY1hK3G https://gcc.godbolt.org/z/634888Y96 I think constexpr string variable that requires heap allocation is currently unimplementable, and constexpr string variable of dynamic storage duration can't be supported by libstdc++-v3 without ABI breakage now. However, should we allow shorter strings to be stored into constexpr basic_string variables? We can do this by always fully initializing the SSO buffer (if it's used) during constant evluation.