[PATCH v2] libstdc++: Fix error handling in std::print

[Jonathan Wakely]

On Tue, 27 Feb 2024 at 15:21, Tim Song wrote:
>
> [print.fun] requires a system_error, but I don't think 
> [ostream.formatted.print] does?

Yeah it looks like I got confused (again) jumping back and forth
between [print.fun] and [ostream.formatted.print]. So we're doing the
right thing, and should just add tests to verify that.

commit e1c689dbeb5b6364eb2a2f0af20ced07b8096b82
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Thu Feb 22 13:06:59 2024

    libstdc++: Test error handling in std::print
    
    The standard requires an exception if std::print fails to write to a
    FILE*. When writing to a std::ostream, failure to format the arguments
    doesn't affect the stream state, but failure to write to the streadm
    sets badbit.
    
    libstdc++-v3/ChangeLog:
    
            * testsuite/27_io/basic_ostream/print/1.cc: Check error
            handling.
            * testsuite/27_io/print/1.cc: Likewise.

diff --git a/libstdc++-v3/testsuite/27_io/basic_ostream/print/1.cc 
b/libstdc++-v3/testsuite/27_io/basic_ostream/print/1.cc
index 71a4daa04c9..cd4b116ac1c 100644
--- a/libstdc++-v3/testsuite/27_io/basic_ostream/print/1.cc
+++ b/libstdc++-v3/testsuite/27_io/basic_ostream/print/1.cc
@@ -103,6 +103,40 @@ test_locale()
   }
 }
 
+void
+test_errors()
+{
+  // Failure to generate output is reported by setting badbit.
+  std::stringstream in(std::ios::in);
+  std::print(in, "{}", "nope"); // No exception here.
+  VERIFY(in.bad());
+#ifdef __cpp_exceptions
+  in.clear();
+  in.exceptions(std::ios::badbit);
+  try
+  {
+    std::print(in, "{}", "nope"); // Should throw now.
+    VERIFY(false);
+  }
+  catch (const std::ios::failure&)
+  {
+  }
+
+  // An exception thrown when formatting the string is propagated
+  // without setting badbit.
+  std::ostringstream out;
+  try
+  {
+    std::vprint_nonunicode(out, "{}", std::make_format_args());
+    VERIFY(false);
+  }
+  catch (const std::format_error&)
+  {
+  }
+  VERIFY(out.good());
+#endif
+}
+
 int main()
 {
   test_print_ostream();
@@ -111,4 +145,5 @@ int main()
   test_print_no_padding();
   test_vprint_nonunicode();
   test_locale();
+  test_errors();
 }
diff --git a/libstdc++-v3/testsuite/27_io/print/1.cc 
b/libstdc++-v3/testsuite/27_io/print/1.cc
index 6a294e0454b..d570f7938be 100644
--- a/libstdc++-v3/testsuite/27_io/print/1.cc
+++ b/libstdc++-v3/testsuite/27_io/print/1.cc
@@ -74,6 +74,21 @@ test_vprint_nonunicode()
   // { dg-output "garbage in . garbage out" }
 }
 
+void
+test_errors()
+{
+#ifdef __cpp_exceptions
+  try
+  {
+    std::print(stdin, "{}", "nope");
+    VERIFY(false);
+  }
+  catch (const std::system_error&)
+  {
+  }
+#endif
+}
+
 int main()
 {
   test_print_default();
@@ -82,4 +97,5 @@ int main()
   test_println_file();
   test_print_raw();
   test_vprint_nonunicode();
+  test_errors();
 }