[gcc r11-9909] libstdc++: Add self-merge check to std::forward_list::merge [PR103853]

[Jonathan Wakely]

https://gcc.gnu.org/g:2089886d5743c20ce0b41d68f0a4cbe097b46704

commit r11-9909-g2089886d5743c20ce0b41d68f0a4cbe097b46704
Author: Pavel I. Kryukov <pavel.kryukov@phystech.edu>
Date:   Thu Jan 6 12:32:36 2022 +0000

    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.
    
    (cherry picked from commit 52ebc2be0990d6d3a46bb716164f9cef6f661021)

Diff:
---
 libstdc++-v3/include/bits/forward_list.tcc         |  5 +++
 .../23_containers/forward_list/operations/merge.cc | 48 ++++++++++++++++++++++
 2 files changed, 53 insertions(+)

diff --git a/libstdc++-v3/include/bits/forward_list.tcc b/libstdc++-v3/include/bits/forward_list.tcc
index bd7a859091f..1145230b910 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 00000000000..0f6f520c33b
--- /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();
+}