diff --git a/clang-tools-extra/include-cleaner/include/clang-include-cleaner/Types.h b/clang-tools-extra/include-cleaner/include/clang-include-cleaner/Types.h
--- a/clang-tools-extra/include-cleaner/include/clang-include-cleaner/Types.h
+++ b/clang-tools-extra/include-cleaner/include/clang-include-cleaner/Types.h
@@ -26,10 +26,12 @@
 #include "clang/Tooling/Inclusions/StandardLibrary.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/DenseMapInfoVariant.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringMap.h"
 #include <memory>
 #include <utility>
+#include <variant>
 #include <vector>
 
 namespace llvm {
diff --git a/flang/include/flang/Optimizer/HLFIR/HLFIROps.h b/flang/include/flang/Optimizer/HLFIR/HLFIROps.h
--- a/flang/include/flang/Optimizer/HLFIR/HLFIROps.h
+++ b/flang/include/flang/Optimizer/HLFIR/HLFIROps.h
@@ -19,6 +19,7 @@
 #include "mlir/IR/PatternMatch.h"
 #include "mlir/Interfaces/InferTypeOpInterface.h"
 #include "mlir/Interfaces/SideEffectInterfaces.h"
+#include <variant>
 
 #include "flang/Optimizer/HLFIR/HLFIROpInterfaces.h.inc"
 #define GET_OP_CLASSES
diff --git a/llvm/include/llvm/ADT/DenseMapInfo.h b/llvm/include/llvm/ADT/DenseMapInfo.h
--- a/llvm/include/llvm/ADT/DenseMapInfo.h
+++ b/llvm/include/llvm/ADT/DenseMapInfo.h
@@ -20,7 +20,6 @@
 #include <tuple>
 #include <type_traits>
 #include <utility>
-#include <variant>
 
 namespace llvm {
 
@@ -234,6 +233,14 @@
                                     SecondInfo::getHashValue(PairVal.second));
   }
 
+  // Expose an additional function intended to be used by other
+  // specializations of DenseMapInfo without needing to know how
+  // to combine hash values manually
+  static unsigned getHashValuePiecewise(const T &First, const U &Second) {
+    return detail::combineHashValue(FirstInfo::getHashValue(First),
+                                    SecondInfo::getHashValue(Second));
+  }
+
   static bool isEqual(const Pair &LHS, const Pair &RHS) {
     return FirstInfo::isEqual(LHS.first, RHS.first) &&
            SecondInfo::isEqual(LHS.second, RHS.second);
@@ -290,52 +297,6 @@
   }
 };
 
-// Provide DenseMapInfo for variants whose all alternatives have DenseMapInfo.
-template <typename... Ts> struct DenseMapInfo<std::variant<Ts...>> {
-  using Variant = std::variant<Ts...>;
-  using FirstT = std::variant_alternative_t<0, Variant>;
-
-  static inline Variant getEmptyKey() {
-    return Variant(std::in_place_index<0>, DenseMapInfo<FirstT>::getEmptyKey());
-  }
-
-  static inline Variant getTombstoneKey() {
-    return Variant(std::in_place_index<0>,
-                   DenseMapInfo<FirstT>::getTombstoneKey());
-  }
-
-  static unsigned getHashValue(const Variant &Val) {
-    return std::visit(
-        [&Val](auto &&Alternative) {
-          using T = std::decay_t<decltype(Alternative)>;
-          // Include index in hash to make sure same value as different
-          // alternatives don't collide.
-          return detail::combineHashValue(
-              DenseMapInfo<size_t>::getHashValue(Val.index()),
-              DenseMapInfo<T>::getHashValue(Alternative));
-        },
-        Val);
-  }
-
-  static bool isEqual(const Variant &LHS, const Variant &RHS) {
-    if (LHS.index() != RHS.index())
-      return false;
-    if (LHS.valueless_by_exception())
-      return true;
-    // We want to dispatch to DenseMapInfo<T>::isEqual(LHS.get(I), RHS.get(I))
-    // We know the types are the same, but std::visit(V, LHS, RHS) doesn't.
-    // We erase the type held in LHS to void*, and dispatch over RHS.
-    const void *ErasedLHS =
-        std::visit([](const auto &LHS) -> const void * { return &LHS; }, LHS);
-    return std::visit(
-        [&](const auto &RHS) -> bool {
-          using T = std::remove_cv_t<std::remove_reference_t<decltype(RHS)>>;
-          return DenseMapInfo<T>::isEqual(*static_cast<const T *>(ErasedLHS),
-                                          RHS);
-        },
-        RHS);
-  }
-};
 } // end namespace llvm
 
 #endif // LLVM_ADT_DENSEMAPINFO_H
diff --git a/llvm/include/llvm/ADT/DenseMapInfoVariant.h b/llvm/include/llvm/ADT/DenseMapInfoVariant.h
new file mode 100644
--- /dev/null
+++ b/llvm/include/llvm/ADT/DenseMapInfoVariant.h
@@ -0,0 +1,71 @@
+//===- DenseMapInfoVariant.h - Type traits for DenseMap<variant> *- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file defines DenseMapInfo traits for DenseMap<std::variant<Ts...>>.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ADT_DENSEMAPINFOVARIANT_H
+#define LLVM_ADT_DENSEMAPINFOVARIANT_H
+
+#include "llvm/ADT/DenseMapInfo.h"
+#include <utility>
+#include <variant>
+
+namespace llvm {
+
+// Provide DenseMapInfo for variants whose all alternatives have DenseMapInfo.
+template <typename... Ts> struct DenseMapInfo<std::variant<Ts...>> {
+  using Variant = std::variant<Ts...>;
+  using FirstT = std::variant_alternative_t<0, Variant>;
+
+  static inline Variant getEmptyKey() {
+    return Variant(std::in_place_index<0>, DenseMapInfo<FirstT>::getEmptyKey());
+  }
+
+  static inline Variant getTombstoneKey() {
+    return Variant(std::in_place_index<0>,
+                   DenseMapInfo<FirstT>::getTombstoneKey());
+  }
+
+  static unsigned getHashValue(const Variant &Val) {
+    return std::visit(
+        [&Val](auto &&Alternative) {
+          using T = std::decay_t<decltype(Alternative)>;
+          // Include index in hash to make sure same value as different
+          // alternatives don't collide.
+          return DenseMapInfo<std::pair<size_t, T>>::getHashValuePiecewise(
+              Val.index(), Alternative);
+        },
+        Val);
+  }
+
+  static bool isEqual(const Variant &LHS, const Variant &RHS) {
+    if (LHS.index() != RHS.index())
+      return false;
+    if (LHS.valueless_by_exception())
+      return true;
+    // We want to dispatch to DenseMapInfo<T>::isEqual(LHS.get(I), RHS.get(I))
+    // We know the types are the same, but std::visit(V, LHS, RHS) doesn't.
+    // We erase the type held in LHS to void*, and dispatch over RHS.
+    const void *ErasedLHS =
+        std::visit([](const auto &LHS) -> const void * { return &LHS; }, LHS);
+    return std::visit(
+        [&](const auto &RHS) -> bool {
+          using T = std::remove_cv_t<std::remove_reference_t<decltype(RHS)>>;
+          return DenseMapInfo<T>::isEqual(*static_cast<const T *>(ErasedLHS),
+                                          RHS);
+        },
+        RHS);
+  }
+};
+
+} // end namespace llvm
+
+#endif // LLVM_ADT_DENSEMAPINFOVARIANT_H
diff --git a/llvm/include/llvm/CodeGen/CallingConvLower.h b/llvm/include/llvm/CodeGen/CallingConvLower.h
--- a/llvm/include/llvm/CodeGen/CallingConvLower.h
+++ b/llvm/include/llvm/CodeGen/CallingConvLower.h
@@ -19,6 +19,8 @@
 #include "llvm/CodeGen/TargetCallingConv.h"
 #include "llvm/IR/CallingConv.h"
 #include "llvm/Support/Alignment.h"
+#include <variant>
+#include <vector>
 
 namespace llvm {
 
diff --git a/llvm/include/llvm/ExecutionEngine/Orc/LLJIT.h b/llvm/include/llvm/ExecutionEngine/Orc/LLJIT.h
--- a/llvm/include/llvm/ExecutionEngine/Orc/LLJIT.h
+++ b/llvm/include/llvm/ExecutionEngine/Orc/LLJIT.h
@@ -22,6 +22,7 @@
 #include "llvm/ExecutionEngine/Orc/ThreadSafeModule.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ThreadPool.h"
+#include <variant>
 
 namespace llvm {
 namespace orc {
diff --git a/llvm/include/llvm/Object/DXContainer.h b/llvm/include/llvm/Object/DXContainer.h
--- a/llvm/include/llvm/Object/DXContainer.h
+++ b/llvm/include/llvm/Object/DXContainer.h
@@ -21,6 +21,7 @@
 #include "llvm/Support/Error.h"
 #include "llvm/Support/MemoryBufferRef.h"
 #include "llvm/TargetParser/Triple.h"
+#include <variant>
 
 namespace llvm {
 namespace object {
diff --git a/llvm/include/llvm/Transforms/Scalar/SROA.h b/llvm/include/llvm/Transforms/Scalar/SROA.h
--- a/llvm/include/llvm/Transforms/Scalar/SROA.h
+++ b/llvm/include/llvm/Transforms/Scalar/SROA.h
@@ -21,6 +21,7 @@
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/IR/PassManager.h"
 #include "llvm/IR/ValueHandle.h"
+#include <variant>
 #include <vector>
 
 namespace llvm {
diff --git a/llvm/unittests/ADT/DenseMapTest.cpp b/llvm/unittests/ADT/DenseMapTest.cpp
--- a/llvm/unittests/ADT/DenseMapTest.cpp
+++ b/llvm/unittests/ADT/DenseMapTest.cpp
@@ -8,6 +8,7 @@
 
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseMapInfo.h"
+#include "llvm/ADT/DenseMapInfoVariant.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include <map>
diff --git a/mlir/include/mlir/IR/AsmState.h b/mlir/include/mlir/IR/AsmState.h
--- a/mlir/include/mlir/IR/AsmState.h
+++ b/mlir/include/mlir/IR/AsmState.h
@@ -20,6 +20,7 @@
 #include "llvm/ADT/StringMap.h"
 
 #include <memory>
+#include <variant>
 
 namespace mlir {
 class AsmResourcePrinter;
diff --git a/mlir/include/mlir/Transforms/SROA.h b/mlir/include/mlir/Transforms/SROA.h
--- a/mlir/include/mlir/Transforms/SROA.h
+++ b/mlir/include/mlir/Transforms/SROA.h
@@ -13,6 +13,7 @@
 #include "mlir/Interfaces/MemorySlotInterfaces.h"
 #include "mlir/Support/LogicalResult.h"
 #include "llvm/ADT/Statistic.h"
+#include <variant>
 
 namespace mlir {