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// Copyright 2023 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef BASE_TYPES_VARIANT_UTIL_H_
#define BASE_TYPES_VARIANT_UTIL_H_
#include <stddef.h>
#include <type_traits>
#include <variant>
#include "base/types/always_false.h"
namespace base {
namespace internal {
template <typename Variant, typename T>
struct VariantIndexOfTypeHelper {
static_assert(AlwaysFalse<Variant>, "Variant must be an std::variant<...>");
};
template <typename... Ts, typename T>
struct VariantIndexOfTypeHelper<std::variant<Ts...>, T> {
static constexpr size_t Index() {
static_assert(std::is_constructible_v<std::variant<LiteralType<Ts>...>,
LiteralType<T>>,
"Variant is not constructible from T");
return std::variant<LiteralType<Ts>...>(LiteralType<T>()).index();
}
// Helper struct; even if `Tag` may not be usable as a literal type, a
// `LiteralType<Tag>` will be.
template <typename Tag>
struct LiteralType {};
};
} // namespace internal
// Returns the 0-based index of `T` in `Variant`'s list of alternative types,
// e.g. given `Variant` == `std::variant<A, B, C>` and `T` == `B`, returns 1.
//
// Note that this helper cannot be used if the list of alternative types
// contains duplicates.
template <typename Variant, typename T>
constexpr size_t VariantIndexOfType() {
return internal::VariantIndexOfTypeHelper<Variant, T>::Index();
}
} // namespace base
#endif // BASE_TYPES_VARIANT_UTIL_H_
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