Factor out slot_map::underlying_swap from slot_map::partition.

See WG21-SG14#131.

SG14/slot_map.h

@@ -278,6 +278,24 @@ class slot_map
         return 1;
     }
 
+    constexpr void underlying_swap(const_iterator cit, const_iterator cjt) {
+        // Swap *it and *jt in the underlying container,
+        // but then fix up their keys so they don't appear to move.
+        auto it_value_index = std::distance(this->cbegin(), cit);
+        auto it = std::next(this->begin(), it_value_index);
+        auto it_reversemap_iter = std::next(reverse_map_.begin(), it_value_index);
+        auto it_slot_iter = std::next(slots_.begin(), *it_reversemap_iter);
+        auto jt_value_index = std::distance(this->cbegin(), cjt);
+        auto jt = std::next(this->begin(), jt_value_index);
+        auto jt_reversemap_iter = std::next(reverse_map_.begin(), jt_value_index);
+        auto jt_slot_iter = std::next(slots_.begin(), *jt_reversemap_iter);
+
+        using std::swap;
+        swap(*it, *jt);
+        swap(*it_slot_iter, *jt_slot_iter);
+        swap(*it_reversemap_iter, *jt_reversemap_iter);
+    }
+
     template<class Pred>
     constexpr iterator partition(const Pred& pred) {
         iterator it = this->begin();
@@ -294,19 +312,7 @@ class slot_map
                 --jt;
                 if (it == jt) return it;
             }
-            // Swap *it and *jt in the underlying container,
-            // but then fix up their keys so they don't appear to move.
-            auto it_value_index = std::distance(values_.begin(), it);
-            auto it_reversemap_iter = std::next(reverse_map_.begin(), it_value_index);
-            auto it_slot_iter = std::next(slots_.begin(), *it_reversemap_iter);
-            auto jt_value_index = std::distance(values_.begin(), jt);
-            auto jt_reversemap_iter = std::next(reverse_map_.begin(), jt_value_index);
-            auto jt_slot_iter = std::next(slots_.begin(), *jt_reversemap_iter);
-
-            using std::swap;
-            swap(*it, *jt);
-            swap(*it_slot_iter, *jt_slot_iter);
-            swap(*it_reversemap_iter, *jt_reversemap_iter);
+            this->underlying_swap(it, jt);
             ++it;
             if (it == jt) return it;
         }