`basic_regex::assign` resets locale -> `imbue` has no effect

[Flamefire]

As per the standard a basic_regex::imbue call resets the stored pattern (i.e. "The regular expression does not match any character sequence after the call.")

Hence a call to assign or the assignment operator is required AFTER imbue.

However the way assign is implemented it resets the locale to the global locale as all end up calling the overload constructing a new regex and assigning it to "this":

llvm-project/libcxx/include/regex

Line 2399 in 320c2ee

return assign(basic_regex(__first, __last, __f));

Hence there is no way to change the locale.

Sample code:

#include <locale>
#include <regex>
#include <cassert>

int test_main(int /*argc*/, char** /*argv*/)
{
    std::locale a("");
    std::locale b = std::locale::classic();
    std::locale::global(a);
    std::regex r;
    assert(r.getloc() == a);
   // Change to b and verify
    assert(r.imbue(b) == a);
    assert(r.getloc() == b);
    r = "pattern";
    assert(r.getloc() == b); // Fires
    assert(r.getloc() == a); // Unexpected success
}