[committed] libstdc++: Check for multiple modifiers in chrono format string [PR110708]

[Jonathan Wakely via Gcc-patches]

Tested x86_64-linux. Pushed to trunk.

This should be backported to gcc-13 too, but it can wait until 13.3
(it's just an accepts-invalid and unlikely to affect anybody in
practice).

-- >8 --

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.
---
 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>
-- 
2.41.0