[gcc r11-10122] libstdc++: Define basic_regex::multiline for non-strict modes

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

https://gcc.gnu.org/g:43d10cce34281e420fa6bdeb9765e83ba79b1aad

commit r11-10122-g43d10cce34281e420fa6bdeb9765e83ba79b1aad
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Oct 1 14:06:42 2021 +0100

    libstdc++: Define basic_regex::multiline for non-strict modes
    
    The regex_constants::multiline constant is defined for non-strict C++11
    and C++14 modes, on the basis that the feature is a DR (even though it
    was really a new feature addition to C++17 and probably shouldn't have
    gone through the issues list).
    
    This makes the basic_regex::multiline constant defined consistently with
    the regex_constants::multiline one.
    
    For strict C++11 and C++14 mode we don't define them, because multiline
    is not a reserved name in those standards.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/regex.h (basic_regex::multiline): Define for
            non-strict C++11 and C++14 modes.
            * include/bits/regex_constants.h (regex_constants::multiline):
            Add _GLIBCXX_RESOLVE_LIB_DEFECTS comment.
    
    (cherry picked from commit 17374dab3eefd282977ad90743c9aff97f2e41ce)

Diff:
---
 libstdc++-v3/include/bits/regex.h           | 2 +-
 libstdc++-v3/include/bits/regex_constants.h | 2 ++
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/regex.h b/libstdc++-v3/include/bits/regex.h
index f794a0f8f00..1ae089c351f 100644
--- a/libstdc++-v3/include/bits/regex.h
+++ b/libstdc++-v3/include/bits/regex.h
@@ -414,7 +414,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       static constexpr flag_type awk = regex_constants::awk;
       static constexpr flag_type grep = regex_constants::grep;
       static constexpr flag_type egrep = regex_constants::egrep;
-#if __cplusplus >= 201703L
+#if __cplusplus >= 201703L || !defined __STRICT_ANSI__
       static constexpr flag_type multiline = regex_constants::multiline;
 #endif
       ///@}
diff --git a/libstdc++-v3/include/bits/regex_constants.h b/libstdc++-v3/include/bits/regex_constants.h
index af689ff93af..0fd2879c817 100644
--- a/libstdc++-v3/include/bits/regex_constants.h
+++ b/libstdc++-v3/include/bits/regex_constants.h
@@ -171,6 +171,8 @@ namespace regex_constants
     static_cast<syntax_option_type>(1 << _S_egrep);
 
 #if __cplusplus >= 201703L || !defined __STRICT_ANSI__
+  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+  // 2503. multiline option should be added to syntax_option_type
   /**
    * Specifies that the `^` anchor matches at the beginning of a line,
    * and the `$` anchor matches at the end of a line, not only at the