annotate win64-msvc/include/kj/common.h @ 143:e95e00bdc3eb

Further win32 build updates

// Copyright (c) 2013-2014 Sandstorm Development Group, Inc. and contributors
// Licensed under the MIT License:
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

// Header that should be #included by everyone.
//
// This defines very simple utilities that are widely applicable.

#ifndef KJ_COMMON_H_
#define KJ_COMMON_H_

#if defined(__GNUC__) && !KJ_HEADER_WARNINGS
#pragma GCC system_header
#endif

#ifndef KJ_NO_COMPILER_CHECK
#if __cplusplus < 201103L && !__CDT_PARSER__ && !_MSC_VER
  #error "This code requires C++11. Either your compiler does not support it or it is not enabled."
  #ifdef __GNUC__
    // Compiler claims compatibility with GCC, so presumably supports -std.
    #error "Pass -std=c++11 on the compiler command line to enable C++11."
  #endif
#endif

#ifdef __GNUC__
  #if __clang__
    #if __clang_major__ < 3 || (__clang_major__ == 3 && __clang_minor__ < 2)
      #warning "This library requires at least Clang 3.2."
    #elif defined(__apple_build_version__) && __apple_build_version__ <= 4250028
      #warning "This library requires at least Clang 3.2.  XCode 4.6's Clang, which claims to be "\
               "version 4.2 (wat?), is actually built from some random SVN revision between 3.1 "\
               "and 3.2.  Unfortunately, it is insufficient for compiling this library.  You can "\
               "download the real Clang 3.2 (or newer) from the Clang web site.  Step-by-step "\
               "instructions can be found in Cap'n Proto's documentation: "\
               "http://kentonv.github.io/capnproto/install.html#clang_32_on_mac_osx"
    #elif __cplusplus >= 201103L && !__has_include(<initializer_list>)
      #warning "Your compiler supports C++11 but your C++ standard library does not.  If your "\
               "system has libc++ installed (as should be the case on e.g. Mac OSX), try adding "\
               "-stdlib=libc++ to your CXXFLAGS."
    #endif
  #else
    #if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 7)
      #warning "This library requires at least GCC 4.7."
    #endif
  #endif
#elif defined(_MSC_VER)
  #if _MSC_VER < 1900
    #error "You need Visual Studio 2015 or better to compile this code."
  #elif !CAPNP_LITE
    // TODO(cleanup): This is KJ, but we're talking about Cap'n Proto.
    #error "As of this writing, Cap'n Proto only supports Visual C++ in 'lite mode'; please #define CAPNP_LITE"
  #endif
#else
  #warning "I don't recognize your compiler.  As of this writing, Clang and GCC are the only "\
           "known compilers with enough C++11 support for this library.  "\
           "#define KJ_NO_COMPILER_CHECK to make this warning go away."
#endif
#endif

#include <stddef.h>
#include <initializer_list>

#if __linux__ && __cplusplus > 201200L
// Hack around stdlib bug with C++14 that exists on some Linux systems.
// Apparently in this mode the C library decides not to define gets() but the C++ library still
// tries to import it into the std namespace. This bug has been fixed at the source but is still
// widely present in the wild e.g. on Ubuntu 14.04.
#undef _GLIBCXX_HAVE_GETS
#endif

#if defined(_MSC_VER)
#include <intrin.h>  // __popcnt
#endif

// =======================================================================================

namespace kj {

typedef unsigned int uint;
typedef unsigned char byte;

// =======================================================================================
// Common macros, especially for common yet compiler-specific features.

// Detect whether RTTI and exceptions are enabled, assuming they are unless we have specific
// evidence to the contrary.  Clients can always define KJ_NO_RTTI or KJ_NO_EXCEPTIONS explicitly
// to override these checks.
#ifdef __GNUC__
  #if !defined(KJ_NO_RTTI) && !__GXX_RTTI
    #define KJ_NO_RTTI 1
  #endif
  #if !defined(KJ_NO_EXCEPTIONS) && !__EXCEPTIONS
    #define KJ_NO_EXCEPTIONS 1
  #endif
#elif defined(_MSC_VER)
  #if !defined(KJ_NO_RTTI) && !defined(_CPPRTTI)
    #define KJ_NO_RTTI 1
  #endif
  #if !defined(KJ_NO_EXCEPTIONS) && !defined(_CPPUNWIND)
    #define KJ_NO_EXCEPTIONS 1
  #endif
#endif

#if !defined(KJ_DEBUG) && !defined(KJ_NDEBUG)
// Heuristically decide whether to enable debug mode.  If DEBUG or NDEBUG is defined, use that.
// Otherwise, fall back to checking whether optimization is enabled.
#if defined(DEBUG) || defined(_DEBUG)
#define KJ_DEBUG
#elif defined(NDEBUG)
#define KJ_NDEBUG
#elif __OPTIMIZE__
#define KJ_NDEBUG
#else
#define KJ_DEBUG
#endif
#endif

#define KJ_DISALLOW_COPY(classname) \
  classname(const classname&) = delete; \
  classname& operator=(const classname&) = delete
// Deletes the implicit copy constructor and assignment operator.

#ifdef __GNUC__
#define KJ_LIKELY(condition) __builtin_expect(condition, true)
#define KJ_UNLIKELY(condition) __builtin_expect(condition, false)
// Branch prediction macros.  Evaluates to the condition given, but also tells the compiler that we
// expect the condition to be true/false enough of the time that it's worth hard-coding branch
// prediction.
#else
#define KJ_LIKELY(condition) (condition)
#define KJ_UNLIKELY(condition) (condition)
#endif

#if defined(KJ_DEBUG) || __NO_INLINE__
#define KJ_ALWAYS_INLINE(prototype) inline prototype
// Don't force inline in debug mode.
#else
#if defined(_MSC_VER)
#define KJ_ALWAYS_INLINE(prototype) __forceinline prototype
#else
#define KJ_ALWAYS_INLINE(prototype) inline prototype __attribute__((always_inline))
#endif
// Force a function to always be inlined.  Apply only to the prototype, not to the definition.
#endif

#if defined(_MSC_VER)
#define KJ_NOINLINE __declspec(noinline)
#else
#define KJ_NOINLINE __attribute__((noinline))
#endif

#if defined(_MSC_VER)
#define KJ_NORETURN(prototype) __declspec(noreturn) prototype
#define KJ_UNUSED
#define KJ_WARN_UNUSED_RESULT
// TODO(msvc): KJ_WARN_UNUSED_RESULT can use _Check_return_ on MSVC, but it's a prefix, so
//   wrapping the whole prototype is needed. http://msdn.microsoft.com/en-us/library/jj159529.aspx
//   Similarly, KJ_UNUSED could use __pragma(warning(suppress:...)), but again that's a prefix.
#else
#define KJ_NORETURN(prototype) prototype __attribute__((noreturn))
#define KJ_UNUSED __attribute__((unused))
#define KJ_WARN_UNUSED_RESULT __attribute__((warn_unused_result))
#endif

#if __clang__
#define KJ_UNUSED_MEMBER __attribute__((unused))
// Inhibits "unused" warning for member variables.  Only Clang produces such a warning, while GCC
// complains if the attribute is set on members.
#else
#define KJ_UNUSED_MEMBER
#endif

#if __clang__
#define KJ_DEPRECATED(reason) \
    __attribute__((deprecated(reason)))
#define KJ_UNAVAILABLE(reason) \
    __attribute__((unavailable(reason)))
#elif __GNUC__
#define KJ_DEPRECATED(reason) \
    __attribute__((deprecated))
#define KJ_UNAVAILABLE(reason)
#else
#define KJ_DEPRECATED(reason)
#define KJ_UNAVAILABLE(reason)
// TODO(msvc): Again, here, MSVC prefers a prefix, __declspec(deprecated).
#endif

namespace _ {  // private

KJ_NORETURN(void inlineRequireFailure(
    const char* file, int line, const char* expectation, const char* macroArgs,
    const char* message = nullptr));

KJ_NORETURN(void unreachable());

}  // namespace _ (private)

#ifdef KJ_DEBUG
#if _MSC_VER
#define KJ_IREQUIRE(condition, ...) \
    if (KJ_LIKELY(condition)); else ::kj::_::inlineRequireFailure( \
        __FILE__, __LINE__, #condition, "" #__VA_ARGS__, __VA_ARGS__)
// Version of KJ_DREQUIRE() which is safe to use in headers that are #included by users.  Used to
// check preconditions inside inline methods.  KJ_IREQUIRE is particularly useful in that
// it will be enabled depending on whether the application is compiled in debug mode rather than
// whether libkj is.
#else
#define KJ_IREQUIRE(condition, ...) \
    if (KJ_LIKELY(condition)); else ::kj::_::inlineRequireFailure( \
        __FILE__, __LINE__, #condition, #__VA_ARGS__, ##__VA_ARGS__)
// Version of KJ_DREQUIRE() which is safe to use in headers that are #included by users.  Used to
// check preconditions inside inline methods.  KJ_IREQUIRE is particularly useful in that
// it will be enabled depending on whether the application is compiled in debug mode rather than
// whether libkj is.
#endif
#else
#define KJ_IREQUIRE(condition, ...)
#endif

#define KJ_IASSERT KJ_IREQUIRE

#define KJ_UNREACHABLE ::kj::_::unreachable();
// Put this on code paths that cannot be reached to suppress compiler warnings about missing
// returns.

#if __clang__
#define KJ_CLANG_KNOWS_THIS_IS_UNREACHABLE_BUT_GCC_DOESNT
#else
#define KJ_CLANG_KNOWS_THIS_IS_UNREACHABLE_BUT_GCC_DOESNT KJ_UNREACHABLE
#endif

// #define KJ_STACK_ARRAY(type, name, size, minStack, maxStack)
//
// Allocate an array, preferably on the stack, unless it is too big.  On GCC this will use
// variable-sized arrays.  For other compilers we could just use a fixed-size array.  `minStack`
// is the stack array size to use if variable-width arrays are not supported.  `maxStack` is the
// maximum stack array size if variable-width arrays *are* supported.
#if __GNUC__ && !__clang__
#define KJ_STACK_ARRAY(type, name, size, minStack, maxStack) \
  size_t name##_size = (size); \
  bool name##_isOnStack = name##_size <= (maxStack); \
  type name##_stack[name##_isOnStack ? size : 0]; \
  ::kj::Array<type> name##_heap = name##_isOnStack ? \
      nullptr : kj::heapArray<type>(name##_size); \
  ::kj::ArrayPtr<type> name = name##_isOnStack ? \
      kj::arrayPtr(name##_stack, name##_size) : name##_heap
#else
#define KJ_STACK_ARRAY(type, name, size, minStack, maxStack) \
  size_t name##_size = (size); \
  bool name##_isOnStack = name##_size <= (minStack); \
  type name##_stack[minStack]; \
  ::kj::Array<type> name##_heap = name##_isOnStack ? \
      nullptr : kj::heapArray<type>(name##_size); \
  ::kj::ArrayPtr<type> name = name##_isOnStack ? \
      kj::arrayPtr(name##_stack, name##_size) : name##_heap
#endif

#define KJ_CONCAT_(x, y) x##y
#define KJ_CONCAT(x, y) KJ_CONCAT_(x, y)
#define KJ_UNIQUE_NAME(prefix) KJ_CONCAT(prefix, __LINE__)
// Create a unique identifier name.  We use concatenate __LINE__ rather than __COUNTER__ so that
// the name can be used multiple times in the same macro.

#if _MSC_VER

#define KJ_CONSTEXPR(...) __VA_ARGS__
// Use in cases where MSVC barfs on constexpr. A replacement keyword (e.g. "const") can be
// provided, or just leave blank to remove the keyword entirely.
//
// TODO(msvc): Remove this hack once MSVC fully supports constexpr.

#ifndef __restrict__
#define __restrict__ __restrict
// TODO(msvc): Would it be better to define a KJ_RESTRICT macro?
#endif

#pragma warning(disable: 4521 4522)
// This warning complains when there are two copy constructors, one for a const reference and
// one for a non-const reference. It is often quite necessary to do this in wrapper templates,
// therefore this warning is dumb and we disable it.

#pragma warning(disable: 4458)
// Warns when a parameter name shadows a class member. Unfortunately my code does this a lot,
// since I don't use a special name format for members.

#else  // _MSC_VER
#define KJ_CONSTEXPR(...) constexpr
#endif

// =======================================================================================
// Template metaprogramming helpers.

template <typename T> struct NoInfer_ { typedef T Type; };
template <typename T> using NoInfer = typename NoInfer_<T>::Type;
// Use NoInfer<T>::Type in place of T for a template function parameter to prevent inference of
// the type based on the parameter value.

template <typename T> struct RemoveConst_ { typedef T Type; };
template <typename T> struct RemoveConst_<const T> { typedef T Type; };
template <typename T> using RemoveConst = typename RemoveConst_<T>::Type;

template <typename> struct IsLvalueReference_ { static constexpr bool value = false; };
template <typename T> struct IsLvalueReference_<T&> { static constexpr bool value = true; };
template <typename T>
inline constexpr bool isLvalueReference() { return IsLvalueReference_<T>::value; }

template <typename T> struct Decay_ { typedef T Type; };
template <typename T> struct Decay_<T&> { typedef typename Decay_<T>::Type Type; };
template <typename T> struct Decay_<T&&> { typedef typename Decay_<T>::Type Type; };
template <typename T> struct Decay_<T[]> { typedef typename Decay_<T*>::Type Type; };
template <typename T> struct Decay_<const T[]> { typedef typename Decay_<const T*>::Type Type; };
template <typename T, size_t s> struct Decay_<T[s]> { typedef typename Decay_<T*>::Type Type; };
template <typename T, size_t s> struct Decay_<const T[s]> { typedef typename Decay_<const T*>::Type Type; };
template <typename T> struct Decay_<const T> { typedef typename Decay_<T>::Type Type; };
template <typename T> struct Decay_<volatile T> { typedef typename Decay_<T>::Type Type; };
template <typename T> using Decay = typename Decay_<T>::Type;

template <bool b> struct EnableIf_;
template <> struct EnableIf_<true> { typedef void Type; };
template <bool b> using EnableIf = typename EnableIf_<b>::Type;
// Use like:
//
//     template <typename T, typename = EnableIf<isValid<T>()>
//     void func(T&& t);

template <typename...> struct VoidSfinae_ { using Type = void; };
template <typename... Ts> using VoidSfinae = typename VoidSfinae_<Ts...>::Type;
// Note: VoidSfinae is std::void_t from C++17.

template <typename T>
T instance() noexcept;
// Like std::declval, but doesn't transform T into an rvalue reference.  If you want that, specify
// instance<T&&>().

struct DisallowConstCopy {
  // Inherit from this, or declare a member variable of this type, to prevent the class from being
  // copyable from a const reference -- instead, it will only be copyable from non-const references.
  // This is useful for enforcing transitive constness of contained pointers.
  //
  // For example, say you have a type T which contains a pointer.  T has non-const methods which
  // modify the value at that pointer, but T's const methods are designed to allow reading only.
  // Unfortunately, if T has a regular copy constructor, someone can simply make a copy of T and
  // then use it to modify the pointed-to value.  However, if T inherits DisallowConstCopy, then
  // callers will only be able to copy non-const instances of T.  Ideally, there is some
  // parallel type ImmutableT which is like a version of T that only has const methods, and can
  // be copied from a const T.
  //
  // Note that due to C++ rules about implicit copy constructors and assignment operators, any
  // type that contains or inherits from a type that disallows const copies will also automatically
  // disallow const copies.  Hey, cool, that's exactly what we want.

  DisallowConstCopy() = default;
  DisallowConstCopy(DisallowConstCopy&) = default;
  DisallowConstCopy(DisallowConstCopy&&) = default;
  DisallowConstCopy& operator=(DisallowConstCopy&) = default;
  DisallowConstCopy& operator=(DisallowConstCopy&&) = default;
};

template <typename T>
struct DisallowConstCopyIfNotConst: public DisallowConstCopy {
  // Inherit from this when implementing a template that contains a pointer to T and which should
  // enforce transitive constness.  If T is a const type, this has no effect.  Otherwise, it is
  // an alias for DisallowConstCopy.
};

template <typename T>
struct DisallowConstCopyIfNotConst<const T> {};

template <typename T> struct IsConst_ { static constexpr bool value = false; };
template <typename T> struct IsConst_<const T> { static constexpr bool value = true; };
template <typename T> constexpr bool isConst() { return IsConst_<T>::value; }

template <typename T> struct EnableIfNotConst_ { typedef T Type; };
template <typename T> struct EnableIfNotConst_<const T>;
template <typename T> using EnableIfNotConst = typename EnableIfNotConst_<T>::Type;

template <typename T> struct EnableIfConst_;
template <typename T> struct EnableIfConst_<const T> { typedef T Type; };
template <typename T> using EnableIfConst = typename EnableIfConst_<T>::Type;

template <typename T> struct RemoveConstOrDisable_ { struct Type; };
template <typename T> struct RemoveConstOrDisable_<const T> { typedef T Type; };
template <typename T> using RemoveConstOrDisable = typename RemoveConstOrDisable_<T>::Type;

template <typename T> struct IsReference_ { static constexpr bool value = false; };
template <typename T> struct IsReference_<T&> { static constexpr bool value = true; };
template <typename T> constexpr bool isReference() { return IsReference_<T>::value; }

template <typename From, typename To>
struct PropagateConst_ { typedef To Type; };
template <typename From, typename To>
struct PropagateConst_<const From, To> { typedef const To Type; };
template <typename From, typename To>
using PropagateConst = typename PropagateConst_<From, To>::Type;

namespace _ {  // private

template <typename T>
T refIfLvalue(T&&);

}  // namespace _ (private)

#define KJ_DECLTYPE_REF(exp) decltype(::kj::_::refIfLvalue(exp))
// Like decltype(exp), but if exp is an lvalue, produces a reference type.
//
//     int i;
//     decltype(i) i1(i);                         // i1 has type int.
//     KJ_DECLTYPE_REF(i + 1) i2(i + 1);          // i2 has type int.
//     KJ_DECLTYPE_REF(i) i3(i);                  // i3 has type int&.
//     KJ_DECLTYPE_REF(kj::mv(i)) i4(kj::mv(i));  // i4 has type int.

template <typename T>
struct CanConvert_ {
  static int sfinae(T);
  static bool sfinae(...);
};

template <typename T, typename U>
constexpr bool canConvert() {
  return sizeof(CanConvert_<U>::sfinae(instance<T>())) == sizeof(int);
}

#if __clang__
template <typename T>
constexpr bool canMemcpy() {
  // Returns true if T can be copied using memcpy instead of using the copy constructor or
  // assignment operator.

  // Clang unhelpfully defines __has_trivial_{copy,assign}(T) to be true if the copy constructor /
  // assign operator are deleted, on the basis that a strict reading of the definition of "trivial"
  // according to the standard says that deleted functions are in fact trivial.  Meanwhile Clang
  // provides these admittedly-better intrinsics, but GCC does not.
  return __is_trivially_constructible(T, const T&) && __is_trivially_assignable(T, const T&);
}
#else
template <typename T>
constexpr bool canMemcpy() {
  // Returns true if T can be copied using memcpy instead of using the copy constructor or
  // assignment operator.

  // GCC defines these to mean what we want them to mean.
  return __has_trivial_copy(T) && __has_trivial_assign(T);
}
#endif

// =======================================================================================
// Equivalents to std::move() and std::forward(), since these are very commonly needed and the
// std header <utility> pulls in lots of other stuff.
//
// We use abbreviated names mv and fwd because these helpers (especially mv) are so commonly used
// that the cost of typing more letters outweighs the cost of being slightly harder to understand
// when first encountered.

template<typename T> constexpr T&& mv(T& t) noexcept { return static_cast<T&&>(t); }
template<typename T> constexpr T&& fwd(NoInfer<T>& t) noexcept { return static_cast<T&&>(t); }

template<typename T> constexpr T cp(T& t) noexcept { return t; }
template<typename T> constexpr T cp(const T& t) noexcept { return t; }
// Useful to force a copy, particularly to pass into a function that expects T&&.

template <typename T, typename U, bool takeT> struct MinType_;
template <typename T, typename U> struct MinType_<T, U, true> { typedef T Type; };
template <typename T, typename U> struct MinType_<T, U, false> { typedef U Type; };

template <typename T, typename U>
using MinType = typename MinType_<T, U, sizeof(T) <= sizeof(U)>::Type;
// Resolves to the smaller of the two input types.

template <typename T, typename U>
inline constexpr auto min(T&& a, U&& b) -> MinType<Decay<T>, Decay<U>> {
  return a < b ? MinType<Decay<T>, Decay<U>>(a) : MinType<Decay<T>, Decay<U>>(b);
}

template <typename T, typename U, bool takeT> struct MaxType_;
template <typename T, typename U> struct MaxType_<T, U, true> { typedef T Type; };
template <typename T, typename U> struct MaxType_<T, U, false> { typedef U Type; };

template <typename T, typename U>
using MaxType = typename MaxType_<T, U, sizeof(T) >= sizeof(U)>::Type;
// Resolves to the larger of the two input types.

template <typename T, typename U>
inline constexpr auto max(T&& a, U&& b) -> MaxType<Decay<T>, Decay<U>> {
  return a > b ? MaxType<Decay<T>, Decay<U>>(a) : MaxType<Decay<T>, Decay<U>>(b);
}

template <typename T, size_t s>
inline constexpr size_t size(T (&arr)[s]) { return s; }
template <typename T>
inline constexpr size_t size(T&& arr) { return arr.size(); }
// Returns the size of the parameter, whether the parameter is a regular C array or a container
// with a `.size()` method.

class MaxValue_ {
private:
  template <typename T>
  inline constexpr T maxSigned() const {
    return (1ull << (sizeof(T) * 8 - 1)) - 1;
  }
  template <typename T>
  inline constexpr T maxUnsigned() const {
    return ~static_cast<T>(0u);
  }

public:
#define _kJ_HANDLE_TYPE(T) \
  inline constexpr operator   signed T() const { return MaxValue_::maxSigned  <  signed T>(); } \
  inline constexpr operator unsigned T() const { return MaxValue_::maxUnsigned<unsigned T>(); }
  _kJ_HANDLE_TYPE(char)
  _kJ_HANDLE_TYPE(short)
  _kJ_HANDLE_TYPE(int)
  _kJ_HANDLE_TYPE(long)
  _kJ_HANDLE_TYPE(long long)
#undef _kJ_HANDLE_TYPE

  inline constexpr operator char() const {
    // `char` is different from both `signed char` and `unsigned char`, and may be signed or
    // unsigned on different platforms.  Ugh.
    return char(-1) < 0 ? MaxValue_::maxSigned<char>()
                        : MaxValue_::maxUnsigned<char>();
  }
};

class MinValue_ {
private:
  template <typename T>
  inline constexpr T minSigned() const {
    return 1ull << (sizeof(T) * 8 - 1);
  }
  template <typename T>
  inline constexpr T minUnsigned() const {
    return 0u;
  }

public:
#define _kJ_HANDLE_TYPE(T) \
  inline constexpr operator   signed T() const { return MinValue_::minSigned  <  signed T>(); } \
  inline constexpr operator unsigned T() const { return MinValue_::minUnsigned<unsigned T>(); }
  _kJ_HANDLE_TYPE(char)
  _kJ_HANDLE_TYPE(short)
  _kJ_HANDLE_TYPE(int)
  _kJ_HANDLE_TYPE(long)
  _kJ_HANDLE_TYPE(long long)
#undef _kJ_HANDLE_TYPE

  inline constexpr operator char() const {
    // `char` is different from both `signed char` and `unsigned char`, and may be signed or
    // unsigned on different platforms.  Ugh.
    return char(-1) < 0 ? MinValue_::minSigned<char>()
                        : MinValue_::minUnsigned<char>();
  }
};

static KJ_CONSTEXPR(const) MaxValue_ maxValue = MaxValue_();
// A special constant which, when cast to an integer type, takes on the maximum possible value of
// that type.  This is useful to use as e.g. a parameter to a function because it will be robust
// in the face of changes to the parameter's type.
//
// `char` is not supported, but `signed char` and `unsigned char` are.

static KJ_CONSTEXPR(const) MinValue_ minValue = MinValue_();
// A special constant which, when cast to an integer type, takes on the minimum possible value
// of that type.  This is useful to use as e.g. a parameter to a function because it will be robust
// in the face of changes to the parameter's type.
//
// `char` is not supported, but `signed char` and `unsigned char` are.

#if __GNUC__
inline constexpr float inf() { return __builtin_huge_valf(); }
inline constexpr float nan() { return __builtin_nanf(""); }

#elif _MSC_VER

// Do what MSVC math.h does
#pragma warning(push)
#pragma warning(disable: 4756)  // "overflow in constant arithmetic"
inline constexpr float inf() { return (float)(1e300 * 1e300); }
#pragma warning(pop)

float nan();
// Unfortunatley, inf() * 0.0f produces a NaN with the sign bit set, whereas our preferred
// canonical NaN should not have the sign bit set. std::numeric_limits<float>::quiet_NaN()
// returns the correct NaN, but we don't want to #include that here. So, we give up and make
// this out-of-line on MSVC.
//
// TODO(msvc): Can we do better?

#else
#error "Not sure how to support your compiler."
#endif

inline constexpr bool isNaN(float f) { return f != f; }
inline constexpr bool isNaN(double f) { return f != f; }

inline int popCount(unsigned int x) {
#if defined(_MSC_VER)
  return __popcnt(x);
  // Note: __popcnt returns unsigned int, but the value is clearly guaranteed to fit into an int
#else
  return __builtin_popcount(x);
#endif
}

// =======================================================================================
// Useful fake containers

template <typename T>
class Range {
public:
  inline constexpr Range(const T& begin, const T& end): begin_(begin), end_(end) {}

  class Iterator {
  public:
    Iterator() = default;
    inline Iterator(const T& value): value(value) {}

    inline const T&  operator* () const { return value; }
    inline const T&  operator[](size_t index) const { return value + index; }
    inline Iterator& operator++() { ++value; return *this; }
    inline Iterator  operator++(int) { return Iterator(value++); }
    inline Iterator& operator--() { --value; return *this; }
    inline Iterator  operator--(int) { return Iterator(value--); }
    inline Iterator& operator+=(ptrdiff_t amount) { value += amount; return *this; }
    inline Iterator& operator-=(ptrdiff_t amount) { value -= amount; return *this; }
    inline Iterator  operator+ (ptrdiff_t amount) const { return Iterator(value + amount); }
    inline Iterator  operator- (ptrdiff_t amount) const { return Iterator(value - amount); }
    inline ptrdiff_t operator- (const Iterator& other) const { return value - other.value; }

    inline bool operator==(const Iterator& other) const { return value == other.value; }
    inline bool operator!=(const Iterator& other) const { return value != other.value; }
    inline bool operator<=(const Iterator& other) const { return value <= other.value; }
    inline bool operator>=(const Iterator& other) const { return value >= other.value; }
    inline bool operator< (const Iterator& other) const { return value <  other.value; }
    inline bool operator> (const Iterator& other) const { return value >  other.value; }

  private:
    T value;
  };

  inline Iterator begin() const { return Iterator(begin_); }
  inline Iterator end() const { return Iterator(end_); }

  inline auto size() const -> decltype(instance<T>() - instance<T>()) { return end_ - begin_; }

private:
  T begin_;
  T end_;
};

template <typename T>
inline constexpr Range<Decay<T>> range(T begin, T end) { return Range<Decay<T>>(begin, end); }
// Returns a fake iterable container containing all values of T from `begin` (inclusive) to `end`
// (exclusive).  Example:
//
//     // Prints 1, 2, 3, 4, 5, 6, 7, 8, 9.
//     for (int i: kj::range(1, 10)) { print(i); }

template <typename T>
inline constexpr Range<size_t> indices(T&& container) {
  // Shortcut for iterating over the indices of a container:
  //
  //     for (size_t i: kj::indices(myArray)) { handle(myArray[i]); }

  return range<size_t>(0, kj::size(container));
}

template <typename T>
class Repeat {
public:
  inline constexpr Repeat(const T& value, size_t count): value(value), count(count) {}

  class Iterator {
  public:
    Iterator() = default;
    inline Iterator(const T& value, size_t index): value(value), index(index) {}

    inline const T&  operator* () const { return value; }
    inline const T&  operator[](ptrdiff_t index) const { return value; }
    inline Iterator& operator++() { ++index; return *this; }
    inline Iterator  operator++(int) { return Iterator(value, index++); }
    inline Iterator& operator--() { --index; return *this; }
    inline Iterator  operator--(int) { return Iterator(value, index--); }
    inline Iterator& operator+=(ptrdiff_t amount) { index += amount; return *this; }
    inline Iterator& operator-=(ptrdiff_t amount) { index -= amount; return *this; }
    inline Iterator  operator+ (ptrdiff_t amount) const { return Iterator(value, index + amount); }
    inline Iterator  operator- (ptrdiff_t amount) const { return Iterator(value, index - amount); }
    inline ptrdiff_t operator- (const Iterator& other) const { return index - other.index; }

    inline bool operator==(const Iterator& other) const { return index == other.index; }
    inline bool operator!=(const Iterator& other) const { return index != other.index; }
    inline bool operator<=(const Iterator& other) const { return index <= other.index; }
    inline bool operator>=(const Iterator& other) const { return index >= other.index; }
    inline bool operator< (const Iterator& other) const { return index <  other.index; }
    inline bool operator> (const Iterator& other) const { return index >  other.index; }

  private:
    T value;
    size_t index;
  };

  inline Iterator begin() const { return Iterator(value, 0); }
  inline Iterator end() const { return Iterator(value, count); }

  inline size_t size() const { return count; }

private:
  T value;
  size_t count;
};

template <typename T>
inline constexpr Repeat<Decay<T>> repeat(T&& value, size_t count) {
  // Returns a fake iterable which contains `count` repeats of `value`.  Useful for e.g. creating
  // a bunch of spaces:  `kj::repeat(' ', indent * 2)`

  return Repeat<Decay<T>>(value, count);
}

// =======================================================================================
// Manually invoking constructors and destructors
//
// ctor(x, ...) and dtor(x) invoke x's constructor or destructor, respectively.

// We want placement new, but we don't want to #include <new>.  operator new cannot be defined in
// a namespace, and defining it globally conflicts with the definition in <new>.  So we have to
// define a dummy type and an operator new that uses it.

namespace _ {  // private
struct PlacementNew {};
}  // namespace _ (private)
} // namespace kj

inline void* operator new(size_t, kj::_::PlacementNew, void* __p) noexcept {
  return __p;
}

inline void operator delete(void*, kj::_::PlacementNew, void* __p) noexcept {}

namespace kj {

template <typename T, typename... Params>
inline void ctor(T& location, Params&&... params) {
  new (_::PlacementNew(), &location) T(kj::fwd<Params>(params)...);
}

template <typename T>
inline void dtor(T& location) {
  location.~T();
}

// =======================================================================================
// Maybe
//
// Use in cases where you want to indicate that a value may be null.  Using Maybe<T&> instead of T*
// forces the caller to handle the null case in order to satisfy the compiler, thus reliably
// preventing null pointer dereferences at runtime.
//
// Maybe<T> can be implicitly constructed from T and from nullptr.  Additionally, it can be
// implicitly constructed from T*, in which case the pointer is checked for nullness at runtime.
// To read the value of a Maybe<T>, do:
//
//    KJ_IF_MAYBE(value, someFuncReturningMaybe()) {
//      doSomething(*value);
//    } else {
//      maybeWasNull();
//    }
//
// KJ_IF_MAYBE's first parameter is a variable name which will be defined within the following
// block.  The variable will behave like a (guaranteed non-null) pointer to the Maybe's value,
// though it may or may not actually be a pointer.
//
// Note that Maybe<T&> actually just wraps a pointer, whereas Maybe<T> wraps a T and a boolean
// indicating nullness.

template <typename T>
class Maybe;

namespace _ {  // private

#if _MSC_VER
  // TODO(msvc): MSVC barfs on noexcept(instance<T&>().~T()) where T = kj::Exception and
  // kj::_::Void. It and every other factorization I've tried produces:
  //   error C2325: 'kj::Blah' unexpected type to the right of '.~': expected 'void'
#define MSVC_NOEXCEPT_DTOR_WORKAROUND(T) __is_nothrow_destructible(T)
#else
#define MSVC_NOEXCEPT_DTOR_WORKAROUND(T) noexcept(instance<T&>().~T())
#endif

template <typename T>
class NullableValue {
  // Class whose interface behaves much like T*, but actually contains an instance of T and a
  // boolean flag indicating nullness.

public:
  inline NullableValue(NullableValue&& other) noexcept(noexcept(T(instance<T&&>())))
      : isSet(other.isSet) {
    if (isSet) {
      ctor(value, kj::mv(other.value));
    }
  }
  inline NullableValue(const NullableValue& other)
      : isSet(other.isSet) {
    if (isSet) {
      ctor(value, other.value);
    }
  }
  inline NullableValue(NullableValue& other)
      : isSet(other.isSet) {
    if (isSet) {
      ctor(value, other.value);
    }
  }
  inline ~NullableValue() noexcept(MSVC_NOEXCEPT_DTOR_WORKAROUND(T)) {
    if (isSet) {
      dtor(value);
    }
  }

  inline T& operator*() & { return value; }
  inline const T& operator*() const & { return value; }
  inline T&& operator*() && { return kj::mv(value); }
  inline const T&& operator*() const && { return kj::mv(value); }
  inline T* operator->() { return &value; }
  inline const T* operator->() const { return &value; }
  inline operator T*() { return isSet ? &value : nullptr; }
  inline operator const T*() const { return isSet ? &value : nullptr; }

  template <typename... Params>
  inline T& emplace(Params&&... params) {
    if (isSet) {
      isSet = false;
      dtor(value);
    }
    ctor(value, kj::fwd<Params>(params)...);
    isSet = true;
    return value;
  }

private:  // internal interface used by friends only
  inline NullableValue() noexcept: isSet(false) {}
  inline NullableValue(T&& t) noexcept(noexcept(T(instance<T&&>())))
      : isSet(true) {
    ctor(value, kj::mv(t));
  }
  inline NullableValue(T& t)
      : isSet(true) {
    ctor(value, t);
  }
  inline NullableValue(const T& t)
      : isSet(true) {
    ctor(value, t);
  }
  inline NullableValue(const T* t)
      : isSet(t != nullptr) {
    if (isSet) ctor(value, *t);
  }
  template <typename U>
  inline NullableValue(NullableValue<U>&& other) noexcept(noexcept(T(instance<U&&>())))
      : isSet(other.isSet) {
    if (isSet) {
      ctor(value, kj::mv(other.value));
    }
  }
  template <typename U>
  inline NullableValue(const NullableValue<U>& other)
      : isSet(other.isSet) {
    if (isSet) {
      ctor(value, other.value);
    }
  }
  template <typename U>
  inline NullableValue(const NullableValue<U&>& other)
      : isSet(other.isSet) {
    if (isSet) {
      ctor(value, *other.ptr);
    }
  }
  inline NullableValue(decltype(nullptr)): isSet(false) {}

  inline NullableValue& operator=(NullableValue&& other) {
    if (&other != this) {
      // Careful about throwing destructors/constructors here.
      if (isSet) {
        isSet = false;
        dtor(value);
      }
      if (other.isSet) {
        ctor(value, kj::mv(other.value));
        isSet = true;
      }
    }
    return *this;
  }

  inline NullableValue& operator=(NullableValue& other) {
    if (&other != this) {
      // Careful about throwing destructors/constructors here.
      if (isSet) {
        isSet = false;
        dtor(value);
      }
      if (other.isSet) {
        ctor(value, other.value);
        isSet = true;
      }
    }
    return *this;
  }

  inline NullableValue& operator=(const NullableValue& other) {
    if (&other != this) {
      // Careful about throwing destructors/constructors here.
      if (isSet) {
        isSet = false;
        dtor(value);
      }
      if (other.isSet) {
        ctor(value, other.value);
        isSet = true;
      }
    }
    return *this;
  }

  inline bool operator==(decltype(nullptr)) const { return !isSet; }
  inline bool operator!=(decltype(nullptr)) const { return isSet; }

private:
  bool isSet;

#if _MSC_VER
#pragma warning(push)
#pragma warning(disable: 4624)
// Warns that the anonymous union has a deleted destructor when T is non-trivial. This warning
// seems broken.
#endif

  union {
    T value;
  };

#if _MSC_VER
#pragma warning(pop)
#endif

  friend class kj::Maybe<T>;
  template <typename U>
  friend NullableValue<U>&& readMaybe(Maybe<U>&& maybe);
};

template <typename T>
inline NullableValue<T>&& readMaybe(Maybe<T>&& maybe) { return kj::mv(maybe.ptr); }
template <typename T>
inline T* readMaybe(Maybe<T>& maybe) { return maybe.ptr; }
template <typename T>
inline const T* readMaybe(const Maybe<T>& maybe) { return maybe.ptr; }
template <typename T>
inline T* readMaybe(Maybe<T&>&& maybe) { return maybe.ptr; }
template <typename T>
inline T* readMaybe(const Maybe<T&>& maybe) { return maybe.ptr; }

template <typename T>
inline T* readMaybe(T* ptr) { return ptr; }
// Allow KJ_IF_MAYBE to work on regular pointers.

}  // namespace _ (private)

#define KJ_IF_MAYBE(name, exp) if (auto name = ::kj::_::readMaybe(exp))

template <typename T>
class Maybe {
  // A T, or nullptr.

  // IF YOU CHANGE THIS CLASS:  Note that there is a specialization of it in memory.h.

public:
  Maybe(): ptr(nullptr) {}
  Maybe(T&& t) noexcept(noexcept(T(instance<T&&>()))): ptr(kj::mv(t)) {}
  Maybe(T& t): ptr(t) {}
  Maybe(const T& t): ptr(t) {}
  Maybe(const T* t) noexcept: ptr(t) {}
  Maybe(Maybe&& other) noexcept(noexcept(T(instance<T&&>()))): ptr(kj::mv(other.ptr)) {}
  Maybe(const Maybe& other): ptr(other.ptr) {}
  Maybe(Maybe& other): ptr(other.ptr) {}

  template <typename U>
  Maybe(Maybe<U>&& other) noexcept(noexcept(T(instance<U&&>()))) {
    KJ_IF_MAYBE(val, kj::mv(other)) {
      ptr = *val;
    }
  }
  template <typename U>
  Maybe(const Maybe<U>& other) {
    KJ_IF_MAYBE(val, other) {
      ptr = *val;
    }
  }

  Maybe(decltype(nullptr)) noexcept: ptr(nullptr) {}

  template <typename... Params>
  inline T& emplace(Params&&... params) {
    // Replace this Maybe's content with a new value constructed by passing the given parametrs to
    // T's constructor. This can be used to initialize a Maybe without copying or even moving a T.
    // Returns a reference to the newly-constructed value.

    return ptr.emplace(kj::fwd<Params>(params)...);
  }

  inline Maybe& operator=(Maybe&& other) { ptr = kj::mv(other.ptr); return *this; }
  inline Maybe& operator=(Maybe& other) { ptr = other.ptr; return *this; }
  inline Maybe& operator=(const Maybe& other) { ptr = other.ptr; return *this; }

  inline bool operator==(decltype(nullptr)) const { return ptr == nullptr; }
  inline bool operator!=(decltype(nullptr)) const { return ptr != nullptr; }

  T& orDefault(T& defaultValue) {
    if (ptr == nullptr) {
      return defaultValue;
    } else {
      return *ptr;
    }
  }
  const T& orDefault(const T& defaultValue) const {
    if (ptr == nullptr) {
      return defaultValue;
    } else {
      return *ptr;
    }
  }

  template <typename Func>
  auto map(Func&& f) & -> Maybe<decltype(f(instance<T&>()))> {
    if (ptr == nullptr) {
      return nullptr;
    } else {
      return f(*ptr);
    }
  }

  template <typename Func>
  auto map(Func&& f) const & -> Maybe<decltype(f(instance<const T&>()))> {
    if (ptr == nullptr) {
      return nullptr;
    } else {
      return f(*ptr);
    }
  }

  template <typename Func>
  auto map(Func&& f) && -> Maybe<decltype(f(instance<T&&>()))> {
    if (ptr == nullptr) {
      return nullptr;
    } else {
      return f(kj::mv(*ptr));
    }
  }

  template <typename Func>
  auto map(Func&& f) const && -> Maybe<decltype(f(instance<const T&&>()))> {
    if (ptr == nullptr) {
      return nullptr;
    } else {
      return f(kj::mv(*ptr));
    }
  }

private:
  _::NullableValue<T> ptr;

  template <typename U>
  friend class Maybe;
  template <typename U>
  friend _::NullableValue<U>&& _::readMaybe(Maybe<U>&& maybe);
  template <typename U>
  friend U* _::readMaybe(Maybe<U>& maybe);
  template <typename U>
  friend const U* _::readMaybe(const Maybe<U>& maybe);
};

template <typename T>
class Maybe<T&>: public DisallowConstCopyIfNotConst<T> {
public:
  Maybe() noexcept: ptr(nullptr) {}
  Maybe(T& t) noexcept: ptr(&t) {}
  Maybe(T* t) noexcept: ptr(t) {}

  template <typename U>
  inline Maybe(Maybe<U&>& other) noexcept: ptr(other.ptr) {}
  template <typename U>
  inline Maybe(const Maybe<const U&>& other) noexcept: ptr(other.ptr) {}
  inline Maybe(decltype(nullptr)) noexcept: ptr(nullptr) {}

  inline Maybe& operator=(T& other) noexcept { ptr = &other; return *this; }
  inline Maybe& operator=(T* other) noexcept { ptr = other; return *this; }
  template <typename U>
  inline Maybe& operator=(Maybe<U&>& other) noexcept { ptr = other.ptr; return *this; }
  template <typename U>
  inline Maybe& operator=(const Maybe<const U&>& other) noexcept { ptr = other.ptr; return *this; }

  inline bool operator==(decltype(nullptr)) const { return ptr == nullptr; }
  inline bool operator!=(decltype(nullptr)) const { return ptr != nullptr; }

  T& orDefault(T& defaultValue) {
    if (ptr == nullptr) {
      return defaultValue;
    } else {
      return *ptr;
    }
  }
  const T& orDefault(const T& defaultValue) const {
    if (ptr == nullptr) {
      return defaultValue;
    } else {
      return *ptr;
    }
  }

  template <typename Func>
  auto map(Func&& f) -> Maybe<decltype(f(instance<T&>()))> {
    if (ptr == nullptr) {
      return nullptr;
    } else {
      return f(*ptr);
    }
  }

private:
  T* ptr;

  template <typename U>
  friend class Maybe;
  template <typename U>
  friend U* _::readMaybe(Maybe<U&>&& maybe);
  template <typename U>
  friend U* _::readMaybe(const Maybe<U&>& maybe);
};

// =======================================================================================
// ArrayPtr
//
// So common that we put it in common.h rather than array.h.

template <typename T>
class ArrayPtr: public DisallowConstCopyIfNotConst<T> {
  // A pointer to an array.  Includes a size.  Like any pointer, it doesn't own the target data,
  // and passing by value only copies the pointer, not the target.

public:
  inline constexpr ArrayPtr(): ptr(nullptr), size_(0) {}
  inline constexpr ArrayPtr(decltype(nullptr)): ptr(nullptr), size_(0) {}
  inline constexpr ArrayPtr(T* ptr, size_t size): ptr(ptr), size_(size) {}
  inline constexpr ArrayPtr(T* begin, T* end): ptr(begin), size_(end - begin) {}
  inline KJ_CONSTEXPR() ArrayPtr(::std::initializer_list<RemoveConstOrDisable<T>> init)
      : ptr(init.begin()), size_(init.size()) {}

  template <size_t size>
  inline constexpr ArrayPtr(T (&native)[size]): ptr(native), size_(size) {}
  // Construct an ArrayPtr from a native C-style array.

  inline operator ArrayPtr<const T>() const {
    return ArrayPtr<const T>(ptr, size_);
  }
  inline ArrayPtr<const T> asConst() const {
    return ArrayPtr<const T>(ptr, size_);
  }

  inline size_t size() const { return size_; }
  inline const T& operator[](size_t index) const {
    KJ_IREQUIRE(index < size_, "Out-of-bounds ArrayPtr access.");
    return ptr[index];
  }
  inline T& operator[](size_t index) {
    KJ_IREQUIRE(index < size_, "Out-of-bounds ArrayPtr access.");
    return ptr[index];
  }

  inline T* begin() { return ptr; }
  inline T* end() { return ptr + size_; }
  inline T& front() { return *ptr; }
  inline T& back() { return *(ptr + size_ - 1); }
  inline const T* begin() const { return ptr; }
  inline const T* end() const { return ptr + size_; }
  inline const T& front() const { return *ptr; }
  inline const T& back() const { return *(ptr + size_ - 1); }

  inline ArrayPtr<const T> slice(size_t start, size_t end) const {
    KJ_IREQUIRE(start <= end && end <= size_, "Out-of-bounds ArrayPtr::slice().");
    return ArrayPtr<const T>(ptr + start, end - start);
  }
  inline ArrayPtr slice(size_t start, size_t end) {
    KJ_IREQUIRE(start <= end && end <= size_, "Out-of-bounds ArrayPtr::slice().");
    return ArrayPtr(ptr + start, end - start);
  }

  inline ArrayPtr<PropagateConst<T, byte>> asBytes() const {
    // Reinterpret the array as a byte array. This is explicitly legal under C++ aliasing
    // rules.
    return { reinterpret_cast<PropagateConst<T, byte>*>(ptr), size_ * sizeof(T) };
  }
  inline ArrayPtr<PropagateConst<T, char>> asChars() const {
    // Reinterpret the array as a char array. This is explicitly legal under C++ aliasing
    // rules.
    return { reinterpret_cast<PropagateConst<T, char>*>(ptr), size_ * sizeof(T) };
  }

  inline bool operator==(decltype(nullptr)) const { return size_ == 0; }
  inline bool operator!=(decltype(nullptr)) const { return size_ != 0; }

  inline bool operator==(const ArrayPtr& other) const {
    if (size_ != other.size_) return false;
    for (size_t i = 0; i < size_; i++) {
      if (ptr[i] != other[i]) return false;
    }
    return true;
  }
  inline bool operator!=(const ArrayPtr& other) const { return !(*this == other); }

private:
  T* ptr;
  size_t size_;
};

template <typename T>
inline constexpr ArrayPtr<T> arrayPtr(T* ptr, size_t size) {
  // Use this function to construct ArrayPtrs without writing out the type name.
  return ArrayPtr<T>(ptr, size);
}

template <typename T>
inline constexpr ArrayPtr<T> arrayPtr(T* begin, T* end) {
  // Use this function to construct ArrayPtrs without writing out the type name.
  return ArrayPtr<T>(begin, end);
}

// =======================================================================================
// Casts

template <typename To, typename From>
To implicitCast(From&& from) {
  // `implicitCast<T>(value)` casts `value` to type `T` only if the conversion is implicit.  Useful
  // for e.g. resolving ambiguous overloads without sacrificing type-safety.
  return kj::fwd<From>(from);
}

template <typename To, typename From>
Maybe<To&> dynamicDowncastIfAvailable(From& from) {
  // If RTTI is disabled, always returns nullptr.  Otherwise, works like dynamic_cast.  Useful
  // in situations where dynamic_cast could allow an optimization, but isn't strictly necessary
  // for correctness.  It is highly recommended that you try to arrange all your dynamic_casts
  // this way, as a dynamic_cast that is necessary for correctness implies a flaw in the interface
  // design.

  // Force a compile error if To is not a subtype of From.  Cross-casting is rare; if it is needed
  // we should have a separate cast function like dynamicCrosscastIfAvailable().
  if (false) {
    kj::implicitCast<From*>(kj::implicitCast<To*>(nullptr));
  }

#if KJ_NO_RTTI
  return nullptr;
#else
  return dynamic_cast<To*>(&from);
#endif
}

template <typename To, typename From>
To& downcast(From& from) {
  // Down-cast a value to a sub-type, asserting that the cast is valid.  In opt mode this is a
  // static_cast, but in debug mode (when RTTI is enabled) a dynamic_cast will be used to verify
  // that the value really has the requested type.

  // Force a compile error if To is not a subtype of From.
  if (false) {
    kj::implicitCast<From*>(kj::implicitCast<To*>(nullptr));
  }

#if !KJ_NO_RTTI
  KJ_IREQUIRE(dynamic_cast<To*>(&from) != nullptr, "Value cannot be downcast() to requested type.");
#endif

  return static_cast<To&>(from);
}

// =======================================================================================
// Defer

namespace _ {  // private

template <typename Func>
class Deferred {
public:
  inline Deferred(Func&& func): func(kj::fwd<Func>(func)), canceled(false) {}
  inline ~Deferred() noexcept(false) { if (!canceled) func(); }
  KJ_DISALLOW_COPY(Deferred);

  // This move constructor is usually optimized away by the compiler.
  inline Deferred(Deferred&& other): func(kj::mv(other.func)), canceled(false) {
    other.canceled = true;
  }
private:
  Func func;
  bool canceled;
};

}  // namespace _ (private)

template <typename Func>
_::Deferred<Func> defer(Func&& func) {
  // Returns an object which will invoke the given functor in its destructor.  The object is not
  // copyable but is movable with the semantics you'd expect.  Since the return type is private,
  // you need to assign to an `auto` variable.
  //
  // The KJ_DEFER macro provides slightly more convenient syntax for the common case where you
  // want some code to run at current scope exit.

  return _::Deferred<Func>(kj::fwd<Func>(func));
}

#define KJ_DEFER(code) auto KJ_UNIQUE_NAME(_kjDefer) = ::kj::defer([&](){code;})
// Run the given code when the function exits, whether by return or exception.

}  // namespace kj

#endif  // KJ_COMMON_H_