* doc/xml/manual/intro.xml: Document LWG 2742 status.
* doc/html/*: Regenerate.
* include/bits/basic_string.h
(basic_string(const T&, size_type, size_type, const Allocator&)): Add
constructor for substring of basic_string_view, as per LWG 2742 but
with additional constraint to fix ambiguity.
* testsuite/21_strings/basic_string/cons/char/9.cc: New test.
* testsuite/21_strings/basic_string/cons/wchar_t/9.cc: New test.
From-SVN: r242416
