Change: Additional rvalue overload for the substr member function and
the corresponding constructor.
Rationale: Improve efficiency of operations on rvalues.
Effect on original feature: Valid C++ 2020 code that created a substring
by calling substr (or the corresponding constructor)
on an xvalue expression with type S
that is a specialization of basic_string
may change meaning in this revision of C++.
[Example 1: std::string s1 ="some long string that forces allocation", s2 = s1;
std::move(s1).substr(10, 5);
assert(s1 == s2); // unspecified, previously guaranteed to be true
std::string s3(std::move(s2), 10, 5);
assert(s1 == s2); // unspecified, previously guaranteed to be true — end example]