libstdc++: Define std::sub_match::swap member function (LWG 3204)

This was approved at the C++ meeting in February.

libstdc++-v3/ChangeLog:

	* include/bits/regex.h (sub_match::swap): New function.
	* testsuite/28_regex/sub_match/lwg3204.cc: New test.

libstdc++-v3/include/bits/regex.h

@@ -1006,6 +1006,16 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       { return this->_M_str().compare({__s, __n}); }
       /// @endcond
 
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3204. sub_match::swap only swaps the base class
+      /// Swap the values of two sub_match objects.
+      void
+      swap(sub_match& __s) noexcept(__is_nothrow_swappable<_BiIter>::value)
+      {
+	this->pair<_BiIter, _BiIter>::swap(__s);
+	std::swap(matched, __s.matched);
+      }
+
     private:
       // Simplified basic_string_view for C++11
       struct __string_view

libstdc++-v3/testsuite/28_regex/sub_match/lwg3204.cc

@@ -0,0 +1,38 @@
+// { dg-do run { target c++11 } }
+#include <regex>
+#include <testsuite_hooks.h>
+
+// LWG 3204. sub_match::swap only swaps the base class
+
+int main()
+{
+  std::sub_match<const char*> a, b;
+  a.matched = true;
+  a.swap(b);
+  VERIFY( ! a.matched );
+  VERIFY( b.matched );
+}
+
+struct iter
+{
+  using value_type = char;
+  using difference_type = long;
+  using pointer = const char*;
+  using reference = const char&;
+  using iterator_category = std::bidirectional_iterator_tag;
+
+  iter();
+  iter(const iter&) noexcept(false);
+
+  iter& operator++();
+  iter operator++(int);
+  iter& operator--();
+  iter operator--(int);
+  reference operator*() const;
+  pointer operator->() const;
+};
+
+using CS = std::csub_match;
+static_assert( noexcept(std::declval<CS&>().swap(std::declval<CS&>())) );
+using IS = std::sub_match<iter>;
+static_assert( ! noexcept(std::declval<IS&>().swap(std::declval<IS&>())) );