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
@@ -290,6 +290,32 @@
   }
 };
 
+namespace detail {
+// Helpers for DenseMapInfo<std::variant>
+
+// Returns a pointer to std::get<Index>(Val), where Index is not constant.
+template <class V, size_t... Indices>
+static const void *variantGetDynamic(const V &Val, int Index,
+                                     std::index_sequence<Indices...>) {
+  const void *Result = nullptr;
+  unsigned I = 0;
+  (((I++ == Index) ? (Result = &std::get<Indices>(Val), true) : false) || ...);
+  return Result;
+}
+
+// Visitor to check equality of values stored in variants.
+// Assumes both values have the same type.
+struct EqVisitor {
+  const void *ErasedRHSVal; // Points to the value stored in B.
+
+  template <class T> bool operator()(const T &LHSVal) const {
+    const T &RHSVal = *reinterpret_cast<const T *>(ErasedRHSVal);
+    return DenseMapInfo<T>::eq(LHSVal, RHSVal);
+  }
+};
+
+} // namespace detail
+
 // Provide DenseMapInfo for variants whose all alternatives have DenseMapInfo.
 template <typename... Ts> struct DenseMapInfo<std::variant<Ts...>> {
   using Variant = std::variant<Ts...>;
@@ -318,7 +344,17 @@
   }
 
   static bool isEqual(const Variant &LHS, const Variant &RHS) {
-    return LHS == 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))
+    // This is surprisingly fiddly:
+    //  - there's no get() for a runtime index (see detail::variantGetDynamic)
+    //  - we know the types are the same, but std::visit(V, LHS, RHS) doesn't
+    // We erase the type held in RHS to void*, and dispatch over LHS.
+    return std::visit<bool>(
+        detail::EqVisitor{
+            dyn_get(RHS, RHS.index(), std::index_sequence_for<Ts...>{})},
+        LHS);
   }
 };
 } // end 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
@@ -690,6 +690,10 @@
 struct B : public A {
   using A::A;
 };
+
+struct AlwaysEqType {
+  bool operator==(const AlwaysEqType &RHS) const { return true; }
+};
 } // namespace
 
 namespace llvm {
@@ -702,6 +706,16 @@
     return LHS.value == RHS.value;
   }
 };
+
+template <> struct DenseMapInfo<AlwaysEqType> {
+  using T = AlwaysEqType;
+  static inline T getEmptyKey() { return {}; }
+  static inline T getTombstoneKey() { return {}; }
+  static unsigned getHashValue(const T &Val) { return 0; }
+  static bool isEqual(const T &LHS, const T &RHS) {
+    return false;
+  }
+};
 } // namespace llvm
 
 namespace {
@@ -725,16 +739,20 @@
 }
 
 TEST(DenseMapCustomTest, VariantSupport) {
-  using variant = std::variant<int, int>;
+  using variant = std::variant<int, int, AlwaysEqType>;
   DenseMap<variant, int> Map;
   variant Keys[] = {
       variant(std::in_place_index<0>, 1),
       variant(std::in_place_index<1>, 1),
+      variant(std::in_place_index<2>),
   };
   Map.try_emplace(Keys[0], 0);
   Map.try_emplace(Keys[1], 1);
   EXPECT_THAT(Map, testing::SizeIs(2));
   EXPECT_NE(DenseMapInfo<variant>::getHashValue(Keys[0]),
             DenseMapInfo<variant>::getHashValue(Keys[1]));
+  // Check that isEqual dispatches to isEqual of underlying type, and not to
+  // operator==.
+  EXPECT_FALSE(DenseMapInfo<variant>::isEqual(Keys[2], Keys[2]));
 }
 } // namespace