diff --git a/src/libslic3r/AStar.hpp b/src/libslic3r/AStar.hpp
index 61c82157b..b35b6a4af 100644
--- a/src/libslic3r/AStar.hpp
+++ b/src/libslic3r/AStar.hpp
@@ -53,7 +53,7 @@ template<class T> struct TracerTraits_
 template<class T>
 using TracerNodeT = typename TracerTraits_<remove_cvref_t<T>>::Node;
 
-constexpr size_t Unassigned = size_t(-1);
+constexpr auto Unassigned = std::numeric_limits<size_t>::max();
 
 template<class Tracer>
 struct QNode // Queue node. Keeps track of scores g, and h
@@ -69,7 +69,11 @@ struct QNode // Queue node. Keeps track of scores g, and h
           size_t              p    = Unassigned,
           float               gval = std::numeric_limits<float>::infinity(),
           float               hval = 0.f)
-        : node{std::move(n)}, parent{p}, queue_id{Unassigned}, g{gval}, h{hval}
+        : node{std::move(n)}
+        , parent{p}
+        , queue_id{InvalidQueueID}
+        , g{gval}
+        , h{hval}
     {}
 };
 
@@ -154,7 +158,7 @@ bool search_route(const Tracer              &tracer,
                     // The cache needs to be updated either way
                     prev_nd = qsucc_nd;
 
-                    if (queue_id == decltype(qopen)::invalid_id())
+                    if (queue_id == InvalidQueueID)
                         // was in closed or unqueued, rescheduling
                         qopen.push(succ_id);
                     else // was in open, updating
diff --git a/src/libslic3r/MutablePriorityQueue.hpp b/src/libslic3r/MutablePriorityQueue.hpp
index b45b8cfff..fd3e7ac2d 100644
--- a/src/libslic3r/MutablePriorityQueue.hpp
+++ b/src/libslic3r/MutablePriorityQueue.hpp
@@ -7,6 +7,10 @@
 #include <limits>
 #include <cstdlib> // adds size_t (without std::)
 
+namespace Slic3r {
+
+constexpr auto InvalidQueueID = std::numeric_limits<size_t>::max();
+
 template<typename T, typename IndexSetter, typename LessPredicate, const bool ResetIndexWhenRemoved = false>
 class MutablePriorityQueue
 {
@@ -41,7 +45,7 @@ public:
 	bool		empty() const						{ return m_heap.empty(); }
 	T&			operator[](std::size_t idx) noexcept { return m_heap[idx]; }
 	const T&	operator[](std::size_t idx) const noexcept { return m_heap[idx]; }
-	static constexpr size_t invalid_id() { return std::numeric_limits<size_t>::max(); }
+    static constexpr size_t invalid_id() { return InvalidQueueID; }
 
 	using iterator		 = typename std::vector<T>::iterator;
 	using const_iterator = typename std::vector<T>::const_iterator;
@@ -291,7 +295,7 @@ public:
 	bool		empty() const						{ return m_heap.empty(); }
 	T&			operator[](std::size_t idx) noexcept { assert(! address::is_padding(idx)); return m_heap[idx]; }
 	const T&    operator[](std::size_t idx) const noexcept { assert(! address::is_padding(idx)); return m_heap[idx]; }
-	static constexpr size_t invalid_id() { return std::numeric_limits<size_t>::max(); }
+    static constexpr size_t invalid_id() { return InvalidQueueID; }
 
 protected:
 	void		update_heap_up(size_t top, size_t bottom);
@@ -450,4 +454,6 @@ inline void MutableSkipHeapPriorityQueue<T, LessPredicate, IndexSetter, blocking
 	}
 }
 
+} // namespace Slic3r
+
 #endif /* slic3r_MutablePriorityQueue_hpp_ */
diff --git a/tests/libslic3r/test_mutable_priority_queue.cpp b/tests/libslic3r/test_mutable_priority_queue.cpp
index 626b0388d..7e31b57c7 100644
--- a/tests/libslic3r/test_mutable_priority_queue.cpp
+++ b/tests/libslic3r/test_mutable_priority_queue.cpp
@@ -4,6 +4,8 @@
 
 #include "libslic3r/MutablePriorityQueue.hpp"
 
+using namespace Slic3r;
+
 // based on https://raw.githubusercontent.com/rollbear/prio_queue/master/self_test.cpp
 // original source Copyright Björn Fahller 2015, Boost Software License, Version 1.0, http://www.boost.org/LICENSE_1_0.txt