[gcc r14-2638] libstdc++: Check for multiple modifiers in chrono format string [PR110708]

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:52bfec7ea0eb0f1a4c5bfa55b0d6b9b6c8808e26

commit r14-2638-g52bfec7ea0eb0f1a4c5bfa55b0d6b9b6c8808e26
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Jul 18 10:36:37 2023 +0100

    libstdc++: Check for multiple modifiers in chrono format string [PR110708]
    
    The logic for handling modified chrono specs like %Ey was just
    restarting the loop after each modifier, and not checking whether we'd
    already seen a modifier.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/110708
            * include/bits/chrono_io.h (__formatter_chrono::_M_parse): Only
            allow a single modifier.
            * testsuite/std/time/format.cc: Check multiple modifiers.

Diff:
---
 libstdc++-v3/include/bits/chrono_io.h     |  5 +++++
 libstdc++-v3/testsuite/std/time/format.cc | 10 ++++++++++
 2 files changed, 15 insertions(+)

diff --git a/libstdc++-v3/include/bits/chrono_io.h b/libstdc++-v3/include/bits/chrono_io.h
index 87caa30b83a..5f06a6d76b4 100644
--- a/libstdc++-v3/include/bits/chrono_io.h
+++ b/libstdc++-v3/include/bits/chrono_io.h
@@ -426,6 +426,11 @@ namespace __format
 		  break;
 		case 'O':
 		case 'E':
+		  if (__mod) [[unlikely]]
+		    {
+		      __allowed_mods = _Mod_none;
+		      break;
+		    }
 		  __mod = __c;
 		  continue;
 		default:
diff --git a/libstdc++-v3/testsuite/std/time/format.cc b/libstdc++-v3/testsuite/std/time/format.cc
index b05e5da1af8..0dc45d58dce 100644
--- a/libstdc++-v3/testsuite/std/time/format.cc
+++ b/libstdc++-v3/testsuite/std/time/format.cc
@@ -68,6 +68,16 @@ test_bad_format_strings()
   // modifier not valid for conversion specifier
   VERIFY( not is_format_string_for("{:%Ea}", t) );
   VERIFY( not is_format_string_for("{:%Oa}", t) );
+
+  // more than one modifier (PR libstdc++/110708)
+  VERIFY( not is_format_string_for("{:%EEc}", t) );
+  VERIFY( not is_format_string_for("{:%EEEc}", t) );
+  VERIFY( not is_format_string_for("{:%OOd}", t) );
+  VERIFY( not is_format_string_for("{:%OOOd}", t) );
+  VERIFY( not is_format_string_for("{:%EEy}", t) );
+  VERIFY( not is_format_string_for("{:%OOy}", t) );
+  VERIFY( not is_format_string_for("{:%OEy}", t) );
+  VERIFY( not is_format_string_for("{:%EOy}", t) );
 }
 
 template<typename I>