libstdc++: Implement "Printing blank lines with println" for C++23

This was recently approved for C++26 at the Tokyo meeting. As suggested
by Stephan T. Lavavej, I'm defining it as an extension for C++23 mode
(when std::print and std::prinln were first added) rather than as a new
C++26 feature. Both MSVC and libc++ have agreed to do this too.

libstdc++-v3/ChangeLog:

	* include/std/ostream (println(ostream&)): Define new overload.
	* include/std/print (println(FILE*), println()): Likewise.
	* testsuite/27_io/basic_ostream/print/2.cc: New test.
	* testsuite/27_io/print/1.cc: Remove unused header.
	* testsuite/27_io/print/3.cc: New test.

libstdc++-v3/include/std/ostream

@@ -995,6 +995,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       std::print(__os, "{}\n",
 		 std::format(__fmt, std::forward<_Args>(__args)...));
     }
+
+  // Defined for C++26, supported as an extension to C++23.
+  inline void println(ostream& __os)
+  {
+#if defined(_WIN32) && !defined(__CYGWIN__)
+    if constexpr (__unicode::__literal_encoding_is_utf8())
+      std::vprint_unicode(__os, "\n", std::make_format_args());
+    else
+#endif
+      __os.put('\n');
+  }
+
 #endif // __cpp_lib_print
 
 #endif // C++11

libstdc++-v3/include/std/print

@@ -136,6 +136,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   vprint_nonunicode(string_view __fmt, format_args __args)
   { std::vprint_nonunicode(stdout, __fmt, __args); }
 
+  // Defined for C++26, supported as an extension to C++23.
+  inline void println(FILE* __stream)
+  {
+#if defined(_WIN32) && !defined(__CYGWIN__)
+    if constexpr (__unicode::__literal_encoding_is_utf8())
+      std::vprint_unicode(__stream, "\n", std::make_format_args());
+    else
+#endif
+      if (std::putc('\n', __stream) == EOF)
+	__throw_system_error(EIO);
+  }
+
+  inline void println() { std::println(stdout); }
+
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace std
 #endif // __cpp_lib_print

libstdc++-v3/testsuite/27_io/basic_ostream/print/2.cc

@@ -0,0 +1,47 @@
+// { dg-additional-options "-lstdc++exp" { target { *-*-mingw* } } }
+// { dg-do run { target c++23 } }
+// { dg-require-fileio "" }
+
+#include <ostream>
+#include <spanstream>
+#include <string_view>
+#include <iostream>
+#include <iomanip>
+#include <testsuite_hooks.h>
+
+void
+test_println_blank_ostream()
+{
+  char buf[4];
+  std::spanstream os(buf);
+  std::println(os);
+  std::string_view txt(os.span());
+  VERIFY( txt == "\n" );
+}
+
+void
+test_errors()
+{
+  // Failure to generate output is reported by setting badbit.
+  std::stringstream in(std::ios::in);
+  std::println(in); // No exception here.
+  VERIFY(in.bad());
+#ifdef __cpp_exceptions
+  in.clear();
+  in.exceptions(std::ios::badbit);
+  try
+  {
+    std::println(in); // Should throw now.
+    VERIFY(false);
+  }
+  catch (const std::ios::failure&)
+  {
+  }
+#endif
+}
+
+int main()
+{
+  test_println_blank_ostream();
+  test_errors();
+}

libstdc++-v3/testsuite/27_io/print/1.cc

@@ -4,7 +4,6 @@
 
 #include <print>
 #include <cstdio>
-#include <spanstream>
 #include <testsuite_hooks.h>
 #include <testsuite_fs.h>
 

libstdc++-v3/testsuite/27_io/print/3.cc

@@ -0,0 +1,53 @@
+// { dg-additional-options "-lstdc++exp" { target { *-*-mingw* } } }
+// { dg-do run { target c++23 } }
+// { dg-require-fileio "" }
+
+#include <print>
+#include <cstdio>
+#include <testsuite_hooks.h>
+#include <testsuite_fs.h>
+
+void
+test_println_blank()
+{
+  std::print("1");
+  std::println();
+  std::println("2");
+  // { dg-output "1\n2" }
+}
+
+void
+test_println_blank_file()
+{
+  __gnu_test::scoped_file f;
+  FILE* strm = std::fopen(f.path.string().c_str(), "w");
+  VERIFY( strm );
+  std::println(strm);
+  std::fclose(strm);
+
+  std::ifstream in(f.path);
+  std::string txt(std::istreambuf_iterator<char>(in), {});
+  VERIFY( txt == "\n" );
+}
+
+void
+test_errors()
+{
+#ifdef __cpp_exceptions
+  try
+  {
+    std::println(stdin);
+    VERIFY(false);
+  }
+  catch (const std::system_error&)
+  {
+  }
+#endif
+}
+
+int main()
+{
+  test_println_blank();
+  test_println_blank_file();
+  test_errors();
+}