diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -72,6 +72,7 @@
 #include <string>
 #include <tuple>
 #include <utility>
+#include <variant>
 
 using namespace llvm;
 
@@ -24943,13 +24944,6 @@
   return DAG.getTokenFactor(SDLoc(N), Aliases);
 }
 
-namespace {
-// TODO: Replace with with std::monostate when we move to C++17.
-struct UnitT { } Unit;
-bool operator==(const UnitT &, const UnitT &) { return true; }
-bool operator!=(const UnitT &, const UnitT &) { return false; }
-} // namespace
-
 // This function tries to collect a bunch of potentially interesting
 // nodes to improve the chains of, all at once. This might seem
 // redundant, as this function gets called when visiting every store
@@ -24971,7 +24965,7 @@
   // space and thus merged with the previous interval at insertion time.
 
   using IMap =
-      llvm::IntervalMap<int64_t, UnitT, 8, IntervalMapHalfOpenInfo<int64_t>>;
+      llvm::IntervalMap<int64_t, std::monostate, 8, IntervalMapHalfOpenInfo<int64_t>>;
   IMap::Allocator A;
   IMap Intervals(A);