src/tint/castable.h

// Copyright 2020 The Tint Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#ifndef SRC_TINT_CASTABLE_H_
#define SRC_TINT_CASTABLE_H_

#include <stdint.h>
#include <functional>
#include <tuple>
#include <utility>

#include "src/tint/traits.h"
#include "src/tint/utils/bitcast.h"
#include "src/tint/utils/crc32.h"
#include "src/tint/utils/defer.h"

#if defined(__clang__)
/// Temporarily disable certain warnings when using Castable API
#define TINT_CASTABLE_PUSH_DISABLE_WARNINGS()                                 \
    _Pragma("clang diagnostic push")                                     /**/ \
        _Pragma("clang diagnostic ignored \"-Wundefined-var-template\"") /**/ \
        static_assert(true, "require extra semicolon")

/// Restore disabled warnings
#define TINT_CASTABLE_POP_DISABLE_WARNINGS() \
    _Pragma("clang diagnostic pop") /**/     \
        static_assert(true, "require extra semicolon")
#else
#define TINT_CASTABLE_PUSH_DISABLE_WARNINGS() static_assert(true, "require extra semicolon")
#define TINT_CASTABLE_POP_DISABLE_WARNINGS() static_assert(true, "require extra semicolon")
#endif

TINT_CASTABLE_PUSH_DISABLE_WARNINGS();

// Forward declarations
namespace tint {
class CastableBase;

/// Ignore is used as a special type used for skipping over types for trait
/// helper functions.
class Ignore {};
}  // namespace tint

namespace tint::detail {
template <typename T>
struct TypeInfoOf;
}  // namespace tint::detail

namespace tint {

/// True if all template types that are not Ignore derive from CastableBase
template <typename... TYPES>
static constexpr bool IsCastable =
    ((traits::IsTypeOrDerived<TYPES, CastableBase> || std::is_same_v<TYPES, Ignore>)&&...) &&
    !(std::is_same_v<TYPES, Ignore> && ...);

/// Helper macro to instantiate the TypeInfo<T> template for `CLASS`.
#define TINT_INSTANTIATE_TYPEINFO(CLASS)                        \
    TINT_CASTABLE_PUSH_DISABLE_WARNINGS();                      \
    template <>                                                 \
    const tint::TypeInfo tint::detail::TypeInfoOf<CLASS>::info{ \
        &tint::detail::TypeInfoOf<CLASS::TrueBase>::info,       \
        #CLASS,                                                 \
        tint::TypeInfo::HashCodeOf<CLASS>(),                    \
        tint::TypeInfo::FullHashCodeOf<CLASS>(),                \
    };                                                          \
    TINT_CASTABLE_POP_DISABLE_WARNINGS()

/// Bit flags that can be passed to the template parameter `FLAGS` of Is() and
/// As().
enum CastFlags {
    /// Disables the static_assert() inside Is(), that compile-time-verifies that
    /// the cast is possible. This flag may be useful for highly-generic template
    /// code that needs to compile for template permutations that generate
    /// impossible casts.
    kDontErrorOnImpossibleCast = 1,
};

/// TypeInfo holds type information for a Castable type.
struct TypeInfo {
    /// The type of a hash code
    using HashCode = uint64_t;

    /// The base class of this type
    const TypeInfo* base;
    /// The type name
    const char* name;
    /// The type hash code
    const HashCode hashcode;
    /// The type hash code bitwise-or'd with all ancestor's hashcodes.
    const HashCode full_hashcode;

    /// @param type the test type info
    /// @returns true if the class with this TypeInfo is of, or derives from the
    /// class with the given TypeInfo.
    inline bool Is(const tint::TypeInfo* type) const {
        // Optimization: Check whether the all the bits of the type's hashcode can
        // be found in the full_hashcode. If a single bit is missing, then we
        // can quickly tell that that this TypeInfo does not derive from `type`.
        if ((full_hashcode & type->hashcode) != type->hashcode) {
            return false;
        }

        // Walk the base types, starting with this TypeInfo, to see if any of the
        // pointers match `type`.
        for (auto* ti = this; ti != nullptr; ti = ti->base) {
            if (ti == type) {
                return true;
            }
        }
        return false;
    }

    /// @returns true if `type` derives from the class `TO`
    /// @param type the object type to test from, which must be, or derive from
    /// type `FROM`.
    /// @see CastFlags
    template <typename TO, typename FROM, int FLAGS = 0>
    static inline bool Is(const tint::TypeInfo* type) {
        constexpr const bool downcast = std::is_base_of<FROM, TO>::value;
        constexpr const bool upcast = std::is_base_of<TO, FROM>::value;
        constexpr const bool nocast = std::is_same<FROM, TO>::value;
        constexpr const bool assert_is_castable = (FLAGS & kDontErrorOnImpossibleCast) == 0;

        static_assert(upcast || downcast || nocast || !assert_is_castable, "impossible cast");

        if (upcast || nocast) {
            return true;
        }

        return type->Is(&Of<std::remove_cv_t<TO>>());
    }

    /// @returns the static TypeInfo for the type T
    template <typename T>
    static const TypeInfo& Of() {
        return detail::TypeInfoOf<std::remove_cv_t<T>>::info;
    }

    /// @returns a compile-time hashcode for the type `T`.
    /// @note the returned hashcode will have at most 2 bits set, as the hashes
    /// are expected to be used in bloom-filters which will quickly saturate when
    /// multiple hashcodes are bitwise-or'd together.
    template <typename T>
    static constexpr HashCode HashCodeOf() {
        static_assert(IsCastable<T>, "T is not Castable");
        static_assert(std::is_same_v<T, std::remove_cv_t<T>>,
                      "Strip const / volatile decorations before calling HashCodeOf");
        /// Use the compiler's "pretty" function name, which includes the template
        /// type, to obtain a unique hash value.
#ifdef _MSC_VER
        constexpr uint32_t crc = utils::CRC32(__FUNCSIG__);
#else
        constexpr uint32_t crc = utils::CRC32(__PRETTY_FUNCTION__);
#endif
        constexpr uint32_t bit_a = (crc & 63);
        constexpr uint32_t bit_b = ((crc >> 6) & 63);
        return (static_cast<HashCode>(1) << bit_a) | (static_cast<HashCode>(1) << bit_b);
    }

    /// @returns the hashcode of the given type, bitwise-or'd with the hashcodes
    /// of all base classes.
    template <typename T>
    static constexpr HashCode FullHashCodeOf() {
        if constexpr (std::is_same_v<T, CastableBase>) {
            return HashCodeOf<CastableBase>();
        } else {
            return HashCodeOf<T>() | FullHashCodeOf<typename T::TrueBase>();
        }
    }

    /// @returns the bitwise-or'd hashcodes of all the types of the tuple `TUPLE`.
    /// @see HashCodeOf
    template <typename TUPLE>
    static constexpr HashCode CombinedHashCodeOfTuple() {
        constexpr auto kCount = std::tuple_size_v<TUPLE>;
        if constexpr (kCount == 0) {
            return 0;
        } else if constexpr (kCount == 1) {
            return HashCodeOf<std::remove_cv_t<std::tuple_element_t<0, TUPLE>>>();
        } else {
            constexpr auto kMid = kCount / 2;
            return CombinedHashCodeOfTuple<traits::SliceTuple<0, kMid, TUPLE>>() |
                   CombinedHashCodeOfTuple<traits::SliceTuple<kMid, kCount - kMid, TUPLE>>();
        }
    }

    /// @returns the bitwise-or'd hashcodes of all the template parameter types.
    /// @see HashCodeOf
    template <typename... TYPES>
    static constexpr HashCode CombinedHashCodeOf() {
        return CombinedHashCodeOfTuple<std::tuple<TYPES...>>();
    }

    /// @returns true if this TypeInfo is of, or derives from any of the types in
    /// `TUPLE`.
    template <typename TUPLE>
    inline bool IsAnyOfTuple() const {
        constexpr auto kCount = std::tuple_size_v<TUPLE>;
        if constexpr (kCount == 0) {
            return false;
        } else if constexpr (kCount == 1) {
            return Is(&Of<std::tuple_element_t<0, TUPLE>>());
        } else if constexpr (kCount == 2) {
            return Is(&Of<std::tuple_element_t<0, TUPLE>>()) ||
                   Is(&Of<std::tuple_element_t<1, TUPLE>>());
        } else if constexpr (kCount == 3) {
            return Is(&Of<std::tuple_element_t<0, TUPLE>>()) ||
                   Is(&Of<std::tuple_element_t<1, TUPLE>>()) ||
                   Is(&Of<std::tuple_element_t<2, TUPLE>>());
        } else {
            // Optimization: Compare the object's hashcode to the bitwise-or of all
            // the tested type's hashcodes. If there's no intersection of bits in
            // the two masks, then we can guarantee that the type is not in `TO`.
            if (full_hashcode & TypeInfo::CombinedHashCodeOfTuple<TUPLE>()) {
                // Possibly one of the types in `TUPLE`.
                // Split the search in two, and scan each block.
                static constexpr auto kMid = kCount / 2;
                return IsAnyOfTuple<traits::SliceTuple<0, kMid, TUPLE>>() ||
                       IsAnyOfTuple<traits::SliceTuple<kMid, kCount - kMid, TUPLE>>();
            }
            return false;
        }
    }

    /// @returns true if this TypeInfo is of, or derives from any of the types in
    /// `TYPES`.
    template <typename... TYPES>
    inline bool IsAnyOf() const {
        return IsAnyOfTuple<std::tuple<TYPES...>>();
    }
};

namespace detail {

/// TypeInfoOf contains a single TypeInfo field for the type T.
/// TINT_INSTANTIATE_TYPEINFO() must be defined in a .cpp file for each type
/// `T`.
template <typename T>
struct TypeInfoOf {
    /// The unique TypeInfo for the type T.
    static const TypeInfo info;
};

/// A placeholder structure used for template parameters that need a default
/// type, but can always be automatically inferred.
struct Infer;

}  // namespace detail

/// @returns true if `obj` is a valid pointer, and is of, or derives from the
/// class `TO`
/// @param obj the object to test from
/// @see CastFlags
template <typename TO, int FLAGS = 0, typename FROM = detail::Infer>
inline bool Is(FROM* obj) {
    if (obj == nullptr) {
        return false;
    }
    return TypeInfo::Is<TO, FROM, FLAGS>(&obj->TypeInfo());
}

/// @returns true if `obj` is a valid pointer, and is of, or derives from the
/// type `TYPE`, and pred(const TYPE*) returns true
/// @param obj the object to test from
/// @param pred predicate function with signature `bool(const TYPE*)` called iff
/// object is of, or derives from the class `TYPE`.
/// @see CastFlags
template <typename TYPE, int FLAGS = 0, typename OBJ = detail::Infer, typename Pred = detail::Infer>
inline bool Is(OBJ* obj, Pred&& pred) {
    return Is<TYPE, FLAGS, OBJ>(obj) && pred(static_cast<std::add_const_t<TYPE>*>(obj));
}

/// @returns true if `obj` is a valid pointer, and is of, or derives from any of
/// the types in `TYPES`.OBJ
/// @param obj the object to query.
template <typename... TYPES, typename OBJ>
inline bool IsAnyOf(OBJ* obj) {
    if (!obj) {
        return false;
    }
    return obj->TypeInfo().template IsAnyOf<TYPES...>();
}

/// @returns obj dynamically cast to the type `TO` or `nullptr` if
/// this object does not derive from `TO`.
/// @param obj the object to cast from
/// @see CastFlags
template <typename TO, int FLAGS = 0, typename FROM = detail::Infer>
inline TO* As(FROM* obj) {
    auto* as_castable = static_cast<CastableBase*>(obj);
    return Is<TO, FLAGS>(obj) ? static_cast<TO*>(as_castable) : nullptr;
}

/// @returns obj dynamically cast to the type `TO` or `nullptr` if
/// this object does not derive from `TO`.
/// @param obj the object to cast from
/// @see CastFlags
template <typename TO, int FLAGS = 0, typename FROM = detail::Infer>
inline const TO* As(const FROM* obj) {
    auto* as_castable = static_cast<const CastableBase*>(obj);
    return Is<TO, FLAGS>(obj) ? static_cast<const TO*>(as_castable) : nullptr;
}

/// CastableBase is the base class for all Castable objects.
/// It is not encouraged to directly derive from CastableBase without using the
/// Castable helper template.
/// @see Castable
class CastableBase {
  public:
    /// Copy constructor
    CastableBase(const CastableBase&) = default;

    /// Destructor
    virtual ~CastableBase() = default;

    /// Copy assignment
    /// @param other the CastableBase to copy
    /// @returns the new CastableBase
    CastableBase& operator=(const CastableBase& other) = default;

    /// @returns the TypeInfo of the object
    virtual const tint::TypeInfo& TypeInfo() const = 0;

    /// @returns true if this object is of, or derives from the class `TO`
    template <typename TO>
    inline bool Is() const {
        return tint::Is<TO>(this);
    }

    /// @returns true if this object is of, or derives from the class `TO` and
    /// pred(const TO*) returns true
    /// @param pred predicate function with signature `bool(const TO*)` called iff
    /// object is of, or derives from the class `TO`.
    template <typename TO, int FLAGS = 0, typename Pred = detail::Infer>
    inline bool Is(Pred&& pred) const {
        return tint::Is<TO, FLAGS>(this, std::forward<Pred>(pred));
    }

    /// @returns true if this object is of, or derives from any of the `TO`
    /// classes.
    template <typename... TO>
    inline bool IsAnyOf() const {
        return tint::IsAnyOf<TO...>(this);
    }

    /// @returns this object dynamically cast to the type `TO` or `nullptr` if
    /// this object does not derive from `TO`.
    /// @see CastFlags
    template <typename TO, int FLAGS = 0>
    inline TO* As() {
        return tint::As<TO, FLAGS>(this);
    }

    /// @returns this object dynamically cast to the type `TO` or `nullptr` if
    /// this object does not derive from `TO`.
    /// @see CastFlags
    template <typename TO, int FLAGS = 0>
    inline const TO* As() const {
        return tint::As<const TO, FLAGS>(this);
    }

  protected:
    CastableBase() = default;
};

/// Castable is a helper to derive `CLASS` from `BASE`, automatically
/// implementing the Is() and As() methods, along with a #Base type alias.
///
/// Example usage:
///
/// ```
/// class Animal : public Castable<Animal> {};
///
/// class Sheep : public Castable<Sheep, Animal> {};
///
/// Sheep* cast_to_sheep(Animal* animal) {
///    // You can query whether a Castable is of the given type with Is<T>():
///    printf("animal is a sheep? %s", animal->Is<Sheep>() ? "yes" : "no");
///
///    // You can always just try the cast with As<T>().
///    // If the object is not of the correct type, As<T>() will return nullptr:
///    return animal->As<Sheep>();
/// }
/// ```
template <typename CLASS, typename BASE = CastableBase>
class Castable : public BASE {
  public:
    // Inherit the `BASE` class constructors.
    using BASE::BASE;

    /// A type alias for `CLASS` to easily access the `BASE` class members.
    /// Base actually aliases to the Castable instead of `BASE` so that you can
    /// use Base in the `CLASS` constructor.
    using Base = Castable;

    /// A type alias for `BASE`.
    using TrueBase = BASE;

    /// @returns the TypeInfo of the object
    const tint::TypeInfo& TypeInfo() const override { return TypeInfo::Of<CLASS>(); }

    /// @returns true if this object is of, or derives from the class `TO`
    /// @see CastFlags
    template <typename TO, int FLAGS = 0>
    inline bool Is() const {
        return tint::Is<TO, FLAGS>(static_cast<const CLASS*>(this));
    }

    /// @returns true if this object is of, or derives from the class `TO` and
    /// pred(const TO*) returns true
    /// @param pred predicate function with signature `bool(const TO*)` called iff
    /// object is of, or derives from the class `TO`.
    template <int FLAGS = 0, typename Pred = detail::Infer>
    inline bool Is(Pred&& pred) const {
        using TO = typename std::remove_pointer<traits::ParameterType<Pred, 0>>::type;
        return tint::Is<TO, FLAGS>(static_cast<const CLASS*>(this), std::forward<Pred>(pred));
    }

    /// @returns true if this object is of, or derives from any of the `TO`
    /// classes.
    template <typename... TO>
    inline bool IsAnyOf() const {
        return tint::IsAnyOf<TO...>(static_cast<const CLASS*>(this));
    }

    /// @returns this object dynamically cast to the type `TO` or `nullptr` if
    /// this object does not derive from `TO`.
    /// @see CastFlags
    template <typename TO, int FLAGS = 0>
    inline TO* As() {
        return tint::As<TO, FLAGS>(this);
    }

    /// @returns this object dynamically cast to the type `TO` or `nullptr` if
    /// this object does not derive from `TO`.
    /// @see CastFlags
    template <typename TO, int FLAGS = 0>
    inline const TO* As() const {
        return tint::As<const TO, FLAGS>(this);
    }
};

namespace detail {
/// <code>typename CastableCommonBaseImpl<TYPES>::type</code> resolves to the
/// common base class for all of TYPES.
template <typename... TYPES>
struct CastableCommonBaseImpl {};

/// Alias to typename CastableCommonBaseImpl<TYPES>::type
template <typename... TYPES>
using CastableCommonBase = typename detail::CastableCommonBaseImpl<TYPES...>::type;

/// CastableCommonBaseImpl template specialization for a single type
template <typename T>
struct CastableCommonBaseImpl<T> {
    /// Common base class of a single type is itself
    using type = T;
};

/// CastableCommonBaseImpl A <-> CastableBase specialization
template <typename A>
struct CastableCommonBaseImpl<A, CastableBase> {
    /// Common base class for A and CastableBase is CastableBase
    using type = CastableBase;
};

/// CastableCommonBaseImpl T <-> Ignore specialization
template <typename T>
struct CastableCommonBaseImpl<T, Ignore> {
    /// Resolves to T as the other type is ignored
    using type = T;
};

/// CastableCommonBaseImpl Ignore <-> T specialization
template <typename T>
struct CastableCommonBaseImpl<Ignore, T> {
    /// Resolves to T as the other type is ignored
    using type = T;
};

/// CastableCommonBaseImpl A <-> B specialization
template <typename A, typename B>
struct CastableCommonBaseImpl<A, B> {
    /// The common base class for A, B and OTHERS
    using type = std::conditional_t<traits::IsTypeOrDerived<A, B>,
                                    B,  // A derives from B
                                    CastableCommonBase<A, typename B::TrueBase>>;
};

/// CastableCommonBaseImpl 3+ types specialization
template <typename A, typename B, typename... OTHERS>
struct CastableCommonBaseImpl<A, B, OTHERS...> {
    /// The common base class for A, B and OTHERS
    using type = CastableCommonBase<CastableCommonBase<A, B>, OTHERS...>;
};

}  // namespace detail

/// Resolves to the common most derived type that each of the types in `TYPES`
/// derives from.
template <typename... TYPES>
using CastableCommonBase = detail::CastableCommonBase<TYPES...>;

/// Default can be used as the default case for a Switch(), when all previous
/// cases failed to match.
///
/// Example:
/// ```
/// Switch(object,
///     [&](TypeA*) { /* ... */ },
///     [&](TypeB*) { /* ... */ },
///     [&](Default) { /* If not TypeA or TypeB */ });
/// ```
struct Default {};

namespace detail {

/// Evaluates to the Switch case type being matched by the switch case function
/// `FN`.
/// @note does not handle the Default case
/// @see Switch().
template <typename FN>
using SwitchCaseType = std::remove_pointer_t<traits::ParameterType<std::remove_reference_t<FN>, 0>>;

/// Evaluates to true if the function `FN` has the signature of a Default case
/// in a Switch().
/// @see Switch().
template <typename FN>
inline constexpr bool IsDefaultCase =
    std::is_same_v<traits::ParameterType<std::remove_reference_t<FN>, 0>, Default>;

/// Searches the list of Switch cases for a Default case, returning the index of
/// the Default case. If the a Default case is not found in the tuple, then -1
/// is returned.
template <typename TUPLE, std::size_t START_IDX = 0>
constexpr int IndexOfDefaultCase() {
    if constexpr (START_IDX < std::tuple_size_v<TUPLE>) {
        return IsDefaultCase<std::tuple_element_t<START_IDX, TUPLE>>
                   ? static_cast<int>(START_IDX)
                   : IndexOfDefaultCase<TUPLE, START_IDX + 1>();
    } else {
        return -1;
    }
}

/// The implementation of Switch() for non-Default cases.
/// Switch splits the cases into two a low and high block of cases, and quickly
/// rules out blocks that cannot match by comparing the TypeInfo::HashCode of
/// the object and the cases in the block. If a block of cases may match the
/// given object's type, then that block is split into two, and the process
/// recurses. When NonDefaultCases() is called with a single case, then As<>
/// will be used to dynamically cast to the case type and if the cast succeeds,
/// then the case handler is called.
/// @returns true if a case handler was found, otherwise false.
template <typename T, typename RETURN_TYPE, typename... CASES>
inline bool NonDefaultCases(T* object,
                            const TypeInfo* type,
                            RETURN_TYPE* result,
                            std::tuple<CASES...>&& cases) {
    using Cases = std::tuple<CASES...>;

    (void)result;  // Not always used, avoid warning.

    static constexpr bool kHasReturnType = !std::is_same_v<RETURN_TYPE, void>;
    static constexpr size_t kNumCases = sizeof...(CASES);

    if constexpr (kNumCases == 0) {
        // No cases. Nothing to do.
        return false;
    } else if constexpr (kNumCases == 1) {  // NOLINT: cpplint doesn't understand
                                            // `else if constexpr`
        // Single case.
        using CaseFunc = std::tuple_element_t<0, Cases>;
        static_assert(!IsDefaultCase<CaseFunc>, "NonDefaultCases called with a Default case");
        // Attempt to dynamically cast the object to the handler type. If that
        // succeeds, call the case handler with the cast object.
        using CaseType = SwitchCaseType<CaseFunc>;
        if (type->Is(&TypeInfo::Of<CaseType>())) {
            auto* ptr = static_cast<CaseType*>(object);
            if constexpr (kHasReturnType) {
                new (result) RETURN_TYPE(static_cast<RETURN_TYPE>(std::get<0>(cases)(ptr)));
            } else {
                std::get<0>(cases)(ptr);
            }
            return true;
        }
        return false;
    } else {
        // Multiple cases.
        // Check the hashcode bits to see if there's any possibility of a case
        // matching in these cases. If there isn't, we can skip all these cases.
        if (type->full_hashcode & TypeInfo::CombinedHashCodeOf<SwitchCaseType<CASES>...>()) {
            // There's a possibility. We need to scan further.
            // Split the cases into two, and recurse.
            constexpr size_t kMid = kNumCases / 2;
            return NonDefaultCases(object, type, result, traits::Slice<0, kMid>(cases)) ||
                   NonDefaultCases(object, type, result,
                                   traits::Slice<kMid, kNumCases - kMid>(cases));
        } else {
            return false;
        }
    }
}

/// The implementation of Switch() for all cases.
/// @see NonDefaultCases
template <typename T, typename RETURN_TYPE, typename... CASES>
inline void SwitchCases(T* object, RETURN_TYPE* result, std::tuple<CASES...>&& cases) {
    using Cases = std::tuple<CASES...>;

    static constexpr int kDefaultIndex = detail::IndexOfDefaultCase<Cases>();
    static constexpr bool kHasDefaultCase = kDefaultIndex >= 0;
    static constexpr bool kHasReturnType = !std::is_same_v<RETURN_TYPE, void>;

    // Static assertions
    static constexpr bool kDefaultIsOK =
        kDefaultIndex == -1 || kDefaultIndex == std::tuple_size_v<Cases> - 1;
    static constexpr bool kReturnIsOK =
        kHasDefaultCase || !kHasReturnType || std::is_constructible_v<RETURN_TYPE>;
    static_assert(kDefaultIsOK, "Default case must be last in Switch()");
    static_assert(kReturnIsOK,
                  "Switch() requires either a Default case or a return type that is either void or "
                  "default-constructable");

    // If the static asserts have fired, don't bother spewing more errors below
    static constexpr bool kAllOK = kDefaultIsOK && kReturnIsOK;
    if constexpr (kAllOK) {
        if (object) {
            auto* type = &object->TypeInfo();
            if constexpr (kHasDefaultCase) {
                // Evaluate non-default cases.
                if (!detail::NonDefaultCases<T>(object, type, result,
                                                traits::Slice<0, kDefaultIndex>(cases))) {
                    // Nothing matched. Evaluate default case.
                    if constexpr (kHasReturnType) {
                        new (result) RETURN_TYPE(
                            static_cast<RETURN_TYPE>(std::get<kDefaultIndex>(cases)({})));
                    } else {
                        std::get<kDefaultIndex>(cases)({});
                    }
                }
            } else {
                if (!detail::NonDefaultCases<T>(object, type, result, std::move(cases))) {
                    // Nothing matched. No default case.
                    if constexpr (kHasReturnType) {
                        new (result) RETURN_TYPE();
                    }
                }
            }
        } else {
            // Object is nullptr, so no cases can match
            if constexpr (kHasDefaultCase) {
                // Evaluate default case.
                if constexpr (kHasReturnType) {
                    new (result)
                        RETURN_TYPE(static_cast<RETURN_TYPE>(std::get<kDefaultIndex>(cases)({})));
                } else {
                    std::get<kDefaultIndex>(cases)({});
                }
            } else {
                // No default case, no case can match.
                if constexpr (kHasReturnType) {
                    new (result) RETURN_TYPE();
                }
            }
        }
    }
}

/// Resolves to T if T is not nullptr_t, otherwise resolves to Ignore.
template <typename T>
using NullptrToIgnore = std::conditional_t<std::is_same_v<T, std::nullptr_t>, Ignore, T>;

/// Resolves to `const TYPE` if any of `CASE_RETURN_TYPES` are const or
/// pointer-to-const, otherwise resolves to TYPE.
template <typename TYPE, typename... CASE_RETURN_TYPES>
using PropagateReturnConst = std::conditional_t<
    // Are any of the pointer-stripped types const?
    (std::is_const_v<std::remove_pointer_t<CASE_RETURN_TYPES>> || ...),
    const TYPE,  // Yes: Apply const to TYPE
    TYPE>;       // No:  Passthrough

/// SwitchReturnTypeImpl is the implementation of SwitchReturnType
template <bool IS_CASTABLE, typename REQUESTED_TYPE, typename... CASE_RETURN_TYPES>
struct SwitchReturnTypeImpl;

/// SwitchReturnTypeImpl specialization for non-castable case types and an
/// explicitly specified return type.
template <typename REQUESTED_TYPE, typename... CASE_RETURN_TYPES>
struct SwitchReturnTypeImpl</*IS_CASTABLE*/ false, REQUESTED_TYPE, CASE_RETURN_TYPES...> {
    /// Resolves to `REQUESTED_TYPE`
    using type = REQUESTED_TYPE;
};

/// SwitchReturnTypeImpl specialization for non-castable case types and an
/// inferred return type.
template <typename... CASE_RETURN_TYPES>
struct SwitchReturnTypeImpl</*IS_CASTABLE*/ false, Infer, CASE_RETURN_TYPES...> {
    /// Resolves to the common type for all the cases return types.
    using type = std::common_type_t<CASE_RETURN_TYPES...>;
};

/// SwitchReturnTypeImpl specialization for castable case types and an
/// explicitly specified return type.
template <typename REQUESTED_TYPE, typename... CASE_RETURN_TYPES>
struct SwitchReturnTypeImpl</*IS_CASTABLE*/ true, REQUESTED_TYPE, CASE_RETURN_TYPES...> {
  public:
    /// Resolves to `const REQUESTED_TYPE*` or `REQUESTED_TYPE*`
    using type = PropagateReturnConst<std::remove_pointer_t<REQUESTED_TYPE>, CASE_RETURN_TYPES...>*;
};

/// SwitchReturnTypeImpl specialization for castable case types and an infered
/// return type.
template <typename... CASE_RETURN_TYPES>
struct SwitchReturnTypeImpl</*IS_CASTABLE*/ true, Infer, CASE_RETURN_TYPES...> {
  private:
    using InferredType =
        CastableCommonBase<detail::NullptrToIgnore<std::remove_pointer_t<CASE_RETURN_TYPES>>...>;

  public:
    /// `const T*` or `T*`, where T is the common base type for all the castable
    /// case types.
    using type = PropagateReturnConst<InferredType, CASE_RETURN_TYPES...>*;
};

/// Resolves to the return type for a Switch() with the requested return type
/// `REQUESTED_TYPE` and case statement return types. If `REQUESTED_TYPE` is
/// Infer then the return type will be inferred from the case return types.
template <typename REQUESTED_TYPE, typename... CASE_RETURN_TYPES>
using SwitchReturnType = typename SwitchReturnTypeImpl<
    IsCastable<NullptrToIgnore<std::remove_pointer_t<CASE_RETURN_TYPES>>...>,
    REQUESTED_TYPE,
    CASE_RETURN_TYPES...>::type;

}  // namespace detail

/// Switch is used to dispatch one of the provided callback case handler
/// functions based on the type of `object` and the parameter type of the case
/// handlers. Switch will sequentially check the type of `object` against each
/// of the switch case handler functions, and will invoke the first case handler
/// function which has a parameter type that matches the object type. When a
/// case handler is matched, it will be called with the single argument of
/// `object` cast to the case handler's parameter type. Switch will invoke at
/// most one case handler. Each of the case functions must have the signature
/// `R(T*)` or `R(const T*)`, where `T` is the type matched by that case and `R`
/// is the return type, consistent across all case handlers.
///
/// An optional default case function with the signature `R(Default)` can be
/// used as the last case. This default case will be called if all previous
/// cases failed to match.
///
/// If `object` is nullptr and a default case is provided, then the default case
/// will be called. If `object` is nullptr and no default case is provided, then
/// no cases will be called.
///
/// Example:
/// ```
/// Switch(object,
///     [&](TypeA*) { /* ... */ },
///     [&](TypeB*) { /* ... */ });
///
/// Switch(object,
///     [&](TypeA*) { /* ... */ },
///     [&](TypeB*) { /* ... */ },
///     [&](Default) { /* Called if object is not TypeA or TypeB */ });
/// ```
///
/// @param object the object who's type is used to
/// @param cases the switch cases
/// @return the value returned by the called case. If no cases matched, then the
/// zero value for the consistent case type.
template <typename RETURN_TYPE = detail::Infer, typename T = CastableBase, typename... CASES>
inline auto Switch(T* object, CASES&&... cases) {
    using ReturnType = detail::SwitchReturnType<RETURN_TYPE, traits::ReturnType<CASES>...>;
    static constexpr bool kHasReturnType = !std::is_same_v<ReturnType, void>;

    if constexpr (kHasReturnType) {
        // Replacement for std::aligned_storage as this is broken on earlier versions of MSVC.
        struct alignas(alignof(ReturnType)) ReturnStorage {
            uint8_t data[sizeof(ReturnType)];
        };
        ReturnStorage storage;
        auto* res = utils::Bitcast<ReturnType*>(&storage);
        TINT_DEFER(res->~ReturnType());
        detail::SwitchCases(object, res, std::forward_as_tuple(std::forward<CASES>(cases)...));
        return *res;
    } else {
        detail::SwitchCases<T, void>(object, nullptr,
                                     std::forward_as_tuple(std::forward<CASES>(cases)...));
    }
}

}  // namespace tint

TINT_CASTABLE_POP_DISABLE_WARNINGS();

#endif  // SRC_TINT_CASTABLE_H_