Fix PR31166: std::inplace_merge seems to be unstable. Thanks to Jan Wilken Dörrie for the suggested fix.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@311952 91177308-0d34-0410-b5e6-96231b3b80d8

include/algorithm

@@ -734,15 +734,15 @@ struct __less<_T1, const _T1>
 };
 
 template <class _Predicate>
-class __negate
+class __invert // invert the sense of a comparison
 {
 private:
     _Predicate __p_;
 public:
-    _LIBCPP_INLINE_VISIBILITY __negate() {}
+    _LIBCPP_INLINE_VISIBILITY __invert() {}
 
     _LIBCPP_INLINE_VISIBILITY
-    explicit __negate(_Predicate __p) : __p_(__p) {}
+    explicit __invert(_Predicate __p) : __p_(__p) {}
 
     template <class _T1>
     _LIBCPP_INLINE_VISIBILITY
@@ -750,7 +750,7 @@ public:
 
     template <class _T1, class _T2>
     _LIBCPP_INLINE_VISIBILITY
-    bool operator()(const _T1& __x, const _T2& __y) {return !__p_(__x, __y);}
+    bool operator()(const _T1& __x, const _T2& __y) {return __p_(__y, __x);}
 };
 
 #ifdef _LIBCPP_DEBUG
@@ -4567,7 +4567,7 @@ __buffered_inplace_merge(_BidirectionalIterator __first, _BidirectionalIterator
         typedef reverse_iterator<value_type*> _Rv;
         __half_inplace_merge(_Rv(__p), _Rv(__buff),
                              _RBi(__middle), _RBi(__first),
-                             _RBi(__last), __negate<_Compare>(__comp));
+                             _RBi(__last), __invert<_Compare>(__comp));
     }
 }
 

test/std/algorithms/alg.sorting/alg.merge/inplace_merge_comp.pass.cpp

@@ -116,6 +116,25 @@ test()
     test<Iter>(1000);
 }
 
+struct less_by_first {
+  template <typename Pair>
+  bool operator()(const Pair& lhs, const Pair& rhs) {
+    return std::less<typename Pair::first_type>()(lhs.first, rhs.first);
+  }
+};
+
+void test_PR31166 ()
+{
+    typedef std::pair<int, int> P;
+    typedef std::vector<P> V;
+    const V vec {{1, 0}, {2, 0}, {2, 1}, {2, 2}, {2, 3}};
+    for ( int i = 0; i < 5; ++i ) {
+        V res = vec;
+        std::inplace_merge(res.begin(), res.begin() + i, res.end(), less_by_first());
+        assert(res == vec);
+    }
+}
+
 int main()
 {
     test<bidirectional_iterator<int*> >();
@@ -146,4 +165,6 @@ int main()
     delete [] ia;
     }
 #endif  // TEST_STD_VER >= 11
+
+    test_PR31166();
 }