libstdc++: Implement std::spanstream for C++23

This implements the <spanstream> header, as proposed for C++23 by P0448R4.

libstdc++-v3/ChangeLog:

	* include/Makefile.am: Add spanstream header.
	* include/Makefile.in: Regenerate.
	* include/precompiled/stdc++.h: Add spanstream header.
	* include/std/version (__cpp_lib_spanstream): Define.
	* include/std/spanstream: New file.
	* testsuite/27_io/spanstream/1.cc: New test.
	* testsuite/27_io/spanstream/version.cc: New test.

2 files changed, 63 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/27_io/spanstream/1.cc b/libstdc++-v3/testsuite/27_io/spanstream/1.cc
new file mode 100644
index 0000000..b66ee60
--- /dev/null
+++ b/libstdc++-v3/testsuite/27_io/spanstream/1.cc
@@ -0,0 +1,53 @@
+// { dg-options "-std=gnu++23" }
+// { dg-do run { target c++23 } }
+
+#include <spanstream>
+
+#ifndef __cpp_lib_spanstream
+# error "Feature-test macro for spanstream missing in <spanstream>"
+#elif __cpp_lib_spanstream != 202106L
+# error "Feature-test macro for spanstream has wrong value in <spanstream>"
+#endif
+
+#include <testsuite_hooks.h>
+
+using std::ispanstream;
+using std::ospanstream;
+using std::span;
+
+void
+test_input()
+{
+  // reading input from a fixed pre-arranged character buffer
+  char input[] = "10 20 30";
+  ispanstream is{span<char>{input}};
+  int i;
+  is >> i;
+  VERIFY(10 == i);
+  is >> i;
+  VERIFY(20 == i);
+  is >> i;
+  VERIFY(30 == i);
+  is >>i;
+  VERIFY(!is);
+}
+
+void
+test_output()
+{
+  // writing to a fixed pre-arranged character buffer
+  char output[30]{}; // zero-initialize array
+  ospanstream os{span<char>{output}};
+  os << 10 << 20 << 30;
+  auto const sp = os.span();
+  VERIFY(6 == sp.size());
+  VERIFY("102030" == std::string(sp.data(), sp.size()));
+  VERIFY(static_cast<void*>(output) == sp.data()); // no copying of underlying data!
+  VERIFY("102030" == std::string(output)); // initialization guaranteed NUL termination
+}
+
+int main()
+{
+  test_input();
+  test_output();
+}
diff --git a/libstdc++-v3/testsuite/27_io/spanstream/version.cc b/libstdc++-v3/testsuite/27_io/spanstream/version.cc
new file mode 100644
index 0000000..6261755
--- /dev/null
+++ b/libstdc++-v3/testsuite/27_io/spanstream/version.cc
@@ -0,0 +1,10 @@
+// { dg-options "-std=gnu++23" }
+// { dg-do compile { target c++23 } }
+
+#include <version>
+
+#ifndef __cpp_lib_spanstream
+# error "Feature-test macro for spanstream missing in <version>"
+#elif __cpp_lib_spanstream != 202106L
+# error "Feature-test macro for spanstream has wrong value in <version>"
+#endif