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[ADT] Add C++20-style llvm::identity #158211

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[ADT] Add C++20-style llvm::identity #158211

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19 changes: 17 additions & 2 deletions llvm/include/llvm/ADT/identity.h
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Original file line number Diff line number Diff line change
Expand Up @@ -15,17 +15,32 @@
#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;

Ty &operator()(Ty &self) const { return self; }
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