[Bug libstdc++/85772] __gnu_cxx::__versa_string doesn't support C++11 allocators

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85772

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Also review it for noexcept, so that for example the copy constructor is
noexcept when using the COW base:

diff --git a/libstdc++-v3/include/ext/rc_string_base.h
b/libstdc++-v3/include/ext/rc_string_base.h
index 46f189331f7..7e027cf6fb6 100644
--- a/libstdc++-v3/include/ext/rc_string_base.h
+++ b/libstdc++-v3/include/ext/rc_string_base.h
@@ -311,7 +311,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION

       __rc_string_base(const _Alloc& __a);

-      __rc_string_base(const __rc_string_base& __rcs);
+      __rc_string_base(const __rc_string_base& __rcs) _GLIBCXX_USE_NOEXCEPT;

 #if __cplusplus >= 201103L
       __rc_string_base(__rc_string_base&& __rcs)
diff --git a/libstdc++-v3/include/ext/vstring.h
b/libstdc++-v3/include/ext/vstring.h
index 605311e9a28..8834bdcc36e 100644
--- a/libstdc++-v3/include/ext/vstring.h
+++ b/libstdc++-v3/include/ext/vstring.h
@@ -143,6 +143,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *  @param  __str  Source string.
        */
       __versa_string(const __versa_string& __str)
+     
_GLIBCXX_NOEXCEPT_IF(std::is_nothrow_copy_constructible<__vstring_base>())
       : __vstring_base(__str) { }

 #if __cplusplus >= 201103L