[gcc r16-9727] libstdc++: Make chrono::parse accept out-of-range values that aren't needed [PR126364]

[Jonathan Wakely]

https://gcc.gnu.org/g:239247823ca6171c919b2fcb8aa3aae93f9913d7

commit r16-9727-g239247823ca6171c919b2fcb8aa3aae93f9913d7
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Jul 29 19:04:37 2026 +0100

    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 <tkaminsk@redhat.com>
    (cherry picked from commit 5a457c7ba71bebfa1c07a4d4bba8a13b03bba2f4)

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

diff --git a/libstdc++-v3/include/bits/chrono_io.h b/libstdc++-v3/include/bits/chrono_io.h
index 6e51a57d4de2..a018f40ef603 100644
--- a/libstdc++-v3/include/bits/chrono_io.h
+++ b/libstdc++-v3/include/bits/chrono_io.h
@@ -4925,22 +4925,22 @@ 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();
+}