libstdc++: Add self-merge check to std::forward_list::merge [PR103853]

This implements the proposed resolution of LWG 3088, so that x.merge(x)
is a no-op, consistent with std::list::merge.

Signed-off-by: Pavel I. Kryukov <pavel.kryukov@phystech.edu>

Co-authored-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	PR libstdc++/103853
	* include/bits/forward_list.tcc (forward_list::merge): Check for
	self-merge.
	* testsuite/23_containers/forward_list/operations/merge.cc: New test.

2 files changed, 53 insertions, 0 deletions

diff --git a/libstdc++-v3/include/bits/forward_list.tcc b/libstdc++-v3/include/bits/forward_list.tcc
index 7d90e82..b0a6545 100644
--- a/libstdc++-v3/include/bits/forward_list.tcc
+++ b/libstdc++-v3/include/bits/forward_list.tcc
@@ -367,6 +367,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       forward_list<_Tp, _Alloc>::
       merge(forward_list&& __list, _Comp __comp)
       {
+	// _GLIBCXX_RESOLVE_LIB_DEFECTS
+	// 3088. forward_list::merge behavior unclear when passed *this
+	if (std::__addressof(__list) == this)
+	  return;
+
 	_Node_base* __node = &this->_M_impl._M_head;
 	while (__node->_M_next && __list._M_impl._M_head._M_next)
 	  {
diff --git a/libstdc++-v3/testsuite/23_containers/forward_list/operations/merge.cc b/libstdc++-v3/testsuite/23_containers/forward_list/operations/merge.cc
new file mode 100644
index 0000000..0f6f520
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/forward_list/operations/merge.cc
@@ -0,0 +1,48 @@
+// { dg-do run { target c++11 } }
+// C++11 23.3.4.6 Operations [forwardlist.ops]
+
+#include <forward_list>
+#include <testsuite_hooks.h>
+
+void
+test_stable()
+{
+  std::forward_list<double> a{1.5, 2.0, 3.5, 4.1};
+  std::forward_list<double> b{1.0, 2.5, 3.0, 4.3, 4.2, 5.0};
+
+  a.merge(b, std::less<int>{});
+
+  // result is sorted with respect to std::less<int>, so 1.0 and 1.5 are
+  // equivalent, and stability guarantee means the element from a comes first.
+  const std::forward_list<double> r { 1.5, 1.0,
+				      2.0, 2.5,
+				      3.5, 3.0,
+				      4.1, 4.3, 4.2,
+				      5.0};
+
+  VERIFY(a == r);
+}
+
+void
+test_lwg3088()
+{
+  // LWG 3088: forward_list::merge behavior unclear when passed *this
+  // PR libstdc++/103853
+  std::forward_list<int> c1{ 1, 2, 3 };
+  const std::forward_list<int> c2 = c1;
+  c1.merge(c1);
+  VERIFY( c1 == c2 );
+  c1.merge(c1, std::less<long>{});
+  VERIFY( c1 == c2 );
+  c1.merge(std::move(c1));
+  VERIFY( c1 == c2 );
+  c1.merge(std::move(c1), std::less<long>{});
+  VERIFY( c1 == c2 );
+}
+
+int
+main()
+{
+  test_stable();
+  test_lwg3088();
+}