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

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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.
This commit is contained in:
Pavel I. Kryukov 2022-01-06 12:32:36 +00:00 committed by Jonathan Wakely
parent ec12ddd1e7
commit 52ebc2be09
2 changed files with 53 additions and 0 deletions
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5
libstdc++-v3/include/bits/forward_list.tcc
View File

@ -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)
{

48
libstdc++-v3/testsuite/23_containers/forward_list/operations/merge.cc Normal file
View File

@ -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();
}