[gcc r14-10008] libstdc++: Implement "Printing blank lines with println" for C++23

[gcc r14-10008] libstdc++: Implement "Printing blank lines with println" for C++23

[Jonathan Wakely]

https://gcc.gnu.org/g:7c2a9dbcc2c1cb1563774068c59d5e09edc59f06

commit r14-10008-g7c2a9dbcc2c1cb1563774068c59d5e09edc59f06
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Mar 21 23:09:14 2024 +0000

    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.

Diff:
---
 libstdc++-v3/include/std/ostream                   | 12 +++++
 libstdc++-v3/include/std/print                     | 14 ++++++
 .../testsuite/27_io/basic_ostream/print/2.cc       | 47 +++++++++++++++++++
 libstdc++-v3/testsuite/27_io/print/1.cc            |  1 -
 libstdc++-v3/testsuite/27_io/print/3.cc            | 53 ++++++++++++++++++++++
 5 files changed, 126 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/ostream b/libstdc++-v3/include/std/ostream
index a136399ad0b..8a21758d0a3 100644
--- a/libstdc++-v3/include/std/ostream
+++ b/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
diff --git a/libstdc++-v3/include/std/print b/libstdc++-v3/include/std/print
index d44033469de..0c259d04de3 100644
--- a/libstdc++-v3/include/std/print
+++ b/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
diff --git a/libstdc++-v3/testsuite/27_io/basic_ostream/print/2.cc b/libstdc++-v3/testsuite/27_io/basic_ostream/print/2.cc
new file mode 100644
index 00000000000..5d1e3efdbf7
--- /dev/null
+++ b/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();
+}
diff --git a/libstdc++-v3/testsuite/27_io/print/1.cc b/libstdc++-v3/testsuite/27_io/print/1.cc
index d570f7938be..f6585d9880a 100644
--- a/libstdc++-v3/testsuite/27_io/print/1.cc
+++ b/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>
 
diff --git a/libstdc++-v3/testsuite/27_io/print/3.cc b/libstdc++-v3/testsuite/27_io/print/3.cc
new file mode 100644
index 00000000000..ffcf7337ce5
--- /dev/null
+++ b/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();
+}