Why doesn't basic_regex support char8_t? - c++

Question

As the title says, the following code snippet generates C2027:
#include<regex>
using namespace std;
int main() {
basic_regex<char8_t> r;
return 0;
}
And the error msg was:
error C2027: use of undefined type 'std::regex_traits<char8_t>'
message : see declaration of 'std::regex_traits<char8_t>'
message : see reference to class template instantiation 'std::basic_regex<char8_t,std::regex_traits<char8_t>>' being compiled
error C2061: syntax error: identifier 'locale_type'
error C2027: use of undefined type 'std::regex_traits<char8_t>'
message : see declaration of 'std::regex_traits<char8_t>'
error C2061: syntax error: identifier 'string_type'
error C3646: 'imbue': unknown override specifier
error C2059: syntax error: '('
error C2334: unexpected token(s) preceding '{'; skipping apparent function body
error C3646: 'getloc': unknown override specifier
error C2059: syntax error: '('
error C2334: unexpected token(s) preceding '{'; skipping apparent function body
error C2079: 'std::basic_regex<char8_t,std::regex_traits<char8_t>>::_Traits' uses undefined class 'std::regex_traits<char8_t>'
But basic_regex<char> works
Can someone tell me why?
char and char8_t look like the same type in terms of their usage...
I'm using VS2021