diff --git a/src/libexpr/pos-idx.hh b/src/libexpr/pos-idx.hh
index e13491560..1d711681f 100644
--- a/src/libexpr/pos-idx.hh
+++ b/src/libexpr/pos-idx.hh
@@ -2,12 +2,15 @@
 
 #include <cinttypes>
 
+#include "util.hh"
+
 namespace nix {
 
 class PosIdx
 {
     friend struct LazyPosAcessors;
     friend class PosTable;
+    friend class std::hash<PosIdx>;
 
 private:
     uint32_t id;
@@ -37,8 +40,28 @@ public:
     {
         return id == other.id;
     }
+
+    size_t hash() const noexcept
+    {
+        size_t h = 854125;
+        hash_combine(h, id);
+        return h;
+    }
 };
 
 inline PosIdx noPos = {};
 
 }
+
+namespace std {
+
+template<>
+struct hash<nix::PosIdx>
+{
+    std::size_t operator()(nix::PosIdx pos) const noexcept
+    {
+        return pos.hash();
+    }
+};
+
+} // namespace std