[committed v2] libstdc++: Make chrono::parse accept out-of-range values that aren't needed [PR126364]

When parsing a time with %R or %T we should ignore out of range hours
and minutes if the type being parsed doesn't need them, e.g. when
parsing a chrono::year_month_day from "2026-07-29 99:99:99" we do not
set failbit, and should continue parsing after the invalid hours and
minutes.

Because we were short circuiting as soon as we saw "99" (in either
field) we didn't parse to the end of the %R or %T field, and then could
set failbit if there were any subsequent characters or flags to parse.

The fix is to only short-circuit when setting failbit, and continue
parsing otherwise.

With this change, we no longer hit the 'break' when __read_unsigned(2)
returns -1 (e.g. because the input was non-numeric) unless we're
parsing a type that needs the %R or %T value. But that's OK, because
__read_unsigned sets failbit when it returns -1 and so the next
__read_chr or __read_unsigned will fail without extracting more
characters, and we'll break there instead. So there's no change in
observable behaviour for non-numeric inputs, only for out-of-range
numeric inputs.

libstdc++-v3/ChangeLog:

	PR libstdc++/126364
	* include/bits/chrono_io.h (_Parser::operator()) <R>: Only break
	early when setting failbit.
	* testsuite/std/time/parse/126364.cc: New test.

Reviewed-by: Tomasz Kamiński <[email protected]>
---

v2: Removed the redundant braces pointed out by Tomasz. Added
test_invalid() to the new test. Added a paragraph to the commit message
about the handling of non-numeric invalid inputs.

Tested x86_64-linux. Pushed to trunk.

I'll backport this, but it can wait until after 16.2 is released.

 libstdc++-v3/include/bits/chrono_io.h         | 16 +++---
 .../testsuite/std/time/parse/126364.cc        | 51 +++++++++++++++++++
 2 files changed, 59 insertions(+), 8 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/std/time/parse/126364.cc

diff --git a/libstdc++-v3/include/bits/chrono_io.h b/libstdc++-v3/include/bits/chrono_io.h
index c5170368f82f..0044b5a2ecc8 100644
--- a/libstdc++-v3/include/bits/chrono_io.h
+++ b/libstdc++-v3/include/bits/chrono_io.h
@@ -4735,22 +4735,22 @@ namespace __detail
 		    {
 		      auto __val = __read_unsigned(2);
 		      if (__val == -1 || __val > 23) [[unlikely]]
-			{
-			  if ((_M_need & _ChronoParts::_TimeOfDay) != 0)
+			if ((_M_need & _ChronoParts::_TimeOfDay) != 0)
+			  {
 			    __err |= ios_base::failbit;
-			  break;
-			}
+			    break;
+			  }
 		      if (!__read_chr(':')) [[unlikely]]
 			break;
 		      __h = hours(__val);
 
 		      __val = __read_unsigned(2);
 		      if (__val == -1 || __val > 60) [[unlikely]]
-			{
-			  if ((_M_need & _ChronoParts::_TimeOfDay) != 0)
+			if ((_M_need & _ChronoParts::_TimeOfDay) != 0)
+			  {
 			    __err |= ios_base::failbit;
-			  break;
-			}
+			    break;
+			  }
 		      __min = minutes(__val);
 
 		      if (__c == 'R')
diff --git a/libstdc++-v3/testsuite/std/time/parse/126364.cc b/libstdc++-v3/testsuite/std/time/parse/126364.cc
new file mode 100644
index 000000000000..43739e46c414
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/time/parse/126364.cc
@@ -0,0 +1,51 @@
+// { dg-do run { target c++20 } }
+
+// Bug 126364 - chrono::from_stream %T and %R short circuit on out of range
+// values even when it doesn't fail the parse
+
+#include <chrono>
+#include <sstream>
+#include <testsuite_hooks.h>
+
+using namespace std::chrono;
+
+static bool check(const char* input, char const* fmt)
+{
+    std::istringstream is(input);
+    year_month_day ymd{};
+    return from_stream(is, fmt, ymd).good() && ymd.ok();
+}
+
+void
+test_pr126364()
+{
+  // Accept out of range numbers for unused hours, minutes, and seconds.
+  VERIFY( check("2026-07-31T25:36:57Z", "%FT%TZ") );  // hour 25 only
+  VERIFY( check("2026-07-31T20:99:57Z", "%FT%TZ") );  // minute 99 only
+  VERIFY( check("2026-07-31T20:36:99Z", "%FT%TZ") );  // second 99 only
+  VERIFY( check("2026-07-31T25:36:57Z", "%FT%T") );   // hour 25 only, no Z
+  VERIFY( check("2026-07-31T20:99:57Z", "%FT%T") );   // minute 99 only, no Z
+  VERIFY( check("2026-07-31T20:36:99Z", "%FT%T") );   // second 99 only, no Z
+  VERIFY( check("2026-07-31 25", "%F %H") );          // %H out of range alone
+  VERIFY( check("2026-07-31 99", "%F %M") );          // %M out of range alone
+}
+
+void
+test_invalid()
+{
+  // Do not accept non-numeric input for unused hours, minutes, and seconds.
+  VERIFY( ! check("2026-07-31 xx", "%F %H") );
+  VERIFY( ! check("2026-07-31 xx", "%F %M") );
+  VERIFY( ! check("2026-07-31 xx", "%F %S") );
+  VERIFY( ! check("2026-07-31 xx:20", "%F %R") );
+  VERIFY( ! check("2026-07-31 10:xx", "%F %R") );
+  VERIFY( ! check("2026-07-31 xx:20:30", "%F %T") );
+  VERIFY( ! check("2026-07-31 10:xx:30", "%F %T") );
+  VERIFY( ! check("2026-07-31 10:20:xx", "%F %T") );
+}
+
+int main()
+{
+  test_pr126364();
+  test_invalid();
+}
-- 
2.55.0