about constexpr to define string& variable

[huidui]

why i can define :
const std::string& s1 = "test";
or
const std::string s1 = "test";
but if i try to define s1 with constexpr, my VS2019 report error
constexpr std::string& s1 ="test'; (due to constexpr only can define reference , pointer, or literal values, so i define string&.
i am confused, why const define pass, but constexpr will fail ?