libstdc++: sstream from string_view (P2495R3) [PR119741]

Add constructors to stringbuf, stringstream, istringstream, and ostringstream,
and a matching overload of str(sv) in each, that take anything convertible to
a string_view in places where the existing ctors and function take a string.
Note this change omits the constraint applied to the istringstream constructor
from string cited as a "drive-by" in P2495R3, as we have determined it is
redundant.

libstdc++-v3/ChangeLog:

	PR libstdc++/119741
	* include/std/sstream: full implementation, really just
	decls, requires clause and plumbing.
	* include/bits/version.def, include/bits/version.h:
	new preprocessor symbol
	__cpp_lib_sstream_from_string_view.
	* testsuite/27_io/basic_stringbuf/cons/char/string_view.cc:
	New tests.
	* testsuite/27_io/basic_istringstream/cons/char/string_view.cc:
	New tests.
	* testsuite/27_io/basic_ostringstream/cons/char/string_view.cc:
	New tests.
	* testsuite/27_io/basic_stringstream/cons/char/string_view.cc:
	New tests.
	* testsuite/27_io/basic_stringbuf/cons/wchar_t/string_view.cc:
	New tests.
	* testsuite/27_io/basic_istringstream/cons/wchar_t/string_view.cc:
	New tests.
	* testsuite/27_io/basic_ostringstream/cons/wchar_t/string_view.cc:
	New tests.
	* testsuite/27_io/basic_stringstream/cons/wchar_t/string_view.cc:
	New tests.

Reviewed-by: Jonathan Wakely

Author: ncm

libstdc++-v3/include/bits/version.def

@@ -649,7 +649,7 @@ ftms = {
   };
   values = {
     v = 1;
-    /* For when there's no gthread.  */
+    // For when there is no gthread.
     cxxmin = 17;
     hosted = yes;
     gthread = no;
@@ -1995,6 +1995,15 @@ ftms = {
   };
 };
 
+ftms = {
+  name = sstream_from_string_view;
+  values = {
+    v = 202306;
+    cxxmin = 26;
+    hosted = yes;
+  };
+};
+
 // Standard test specifications.
 stds[97] = ">= 199711L";
 stds[03] = ">= 199711L";

libstdc++-v3/include/bits/version.h

@@ -2233,4 +2233,14 @@
 #endif /* !defined(__cpp_lib_polymorphic) && defined(__glibcxx_want_polymorphic) */
 #undef __glibcxx_want_polymorphic
 
+#if !defined(__cpp_lib_sstream_from_string_view)
+# if (__cplusplus >  202302L) && _GLIBCXX_HOSTED
+#  define __glibcxx_sstream_from_string_view 202306L
+#  if defined(__glibcxx_want_all) || defined(__glibcxx_want_sstream_from_string_view)
+#   define __cpp_lib_sstream_from_string_view 202306L
+#  endif
+# endif
+#endif /* !defined(__cpp_lib_sstream_from_string_view) && defined(__glibcxx_want_sstream_from_string_view) */
+#undef __glibcxx_want_sstream_from_string_view
+
 #undef __glibcxx_want_all