diff --git a/llvm/include/llvm/ADT/identity.h b/llvm/include/llvm/ADT/identity.h
index 88d033fc01141..9d599c6c192d8 100644
--- a/llvm/include/llvm/ADT/identity.h
+++ b/llvm/include/llvm/ADT/identity.h
@@ -15,10 +15,12 @@
 #ifndef LLVM_ADT_IDENTITY_H
 #define LLVM_ADT_IDENTITY_H
 
+#include <utility>
+
 namespace llvm {
 
-// Similar to `std::identity` from C++20.
-template <class Ty> struct identity {
+// Our legacy llvm::identity, not quite the same as std::identity.
+template <class Ty = void> struct identity {
   using is_transparent = void;
   using argument_type = Ty;
 
@@ -26,6 +28,19 @@ template <class Ty> struct identity {
   const Ty &operator()(const Ty &self) const { return self; }
 };
 
+// Forward-ported from C++20.
+//
+// While we are migrating from the legacy version, we must refer to this
+// template as identity<>.  Once the legacy version is removed, we can
+// make this a non-template struct and remove the <>.
+template <> struct identity<void> {
+  using is_transparent = void;
+
+  template <typename T> constexpr T &&operator()(T &&self) const {
+    return std::forward<T>(self);
+  }
+};
+
 } // namespace llvm
 
 #endif // LLVM_ADT_IDENTITY_H