[ADT] Add C++20-style llvm::identity

[kazutakahirata]

Currently, our llvm:identity is not quite like std::identity from
C++20. Ours is a template struct while std::identity is a
non-template struct with templatized operator(). This difference
means that we cannot mechanically replace llvm::identity with
std::identity when we switch to C++20 in the future.

This patch implements llvm::identity that actually behaves like
std::identity. The only difference is that we must refer to it as
llvm::identity<> with <> at the end.

Once this patch lands, I'm planning to migrate users of llvm::identity
to the new style. There aren't that many:

• 4 instances of SparseSet<...>
• 3 instances of SparseMultiSet<...>
• about 50 instances of IndexedMap<...>

[llvmbot]

@llvm/pr-subscribers-llvm-adt

Author: Kazu Hirata (kazutakahirata)

Changes

Currently, our llvm:identity<T> is not quite like std::identity from
C++20. Ours is a template struct while std::identity is a
non-template struct with templatized operator(). This difference
means that we cannot mechanically replace llvm::identity with
std::identity when we switch to C++20 in the future.

This patch implements llvm::identity that actually behaves like
std::identity. The only difference is that we must refer to it as
llvm::identity<> with <> at the end.

Once this patch lands, I'm planning to migrate users of llvm::identity
to the new style. There aren't that many:

• 4 instances of SparseSet<...>
• 3 instances of SparseMultiSet<...>
• about 50 instances of IndexedMap<...>

---

Full diff: https://github.com/llvm/llvm-project/pull/158211.diff

1 Files Affected:

• (modified) llvm/include/llvm/ADT/identity.h (+17-2)

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