VC++错误，编译时顺序操作

Question

我正在尝试为编译时积分序列实现一些功能。下面是操作or的实现：

#include <cstdint>
#include <utility>

using UInt32 = std::uint32_t;

template<UInt32... s>
using Sequence = std::integer_sequence<UInt32, s...>;

template<UInt32>
constexpr bool find([[maybe_unused]] Sequence<> a = {}) noexcept {
    return false;
}

template<UInt32 x, UInt32 v, UInt32... s>
constexpr bool find([[maybe_unused]] Sequence<v, s...> a = {}) noexcept {
    if constexpr (v == x) {
        return true;
    } else {
        return find<x, s...>();
    }
}

constexpr auto operator|(Sequence<>, Sequence<>) noexcept {
    return Sequence<>{};
}

template<UInt32 v, UInt32... s>
constexpr auto operator|(Sequence<v, s...>, Sequence<>) noexcept {
    return Sequence<v, s...>{};
}

template<UInt32... s, UInt32 x, UInt32... t>
constexpr auto operator|(Sequence<s...>, Sequence<x, t...>) noexcept {
    if constexpr (find<x, s...>()) {
        return Sequence<s...>{} | Sequence<t...>{};
    } else {
        return Sequence<s..., x>{} | Sequence<t...>{}; // error C2679
    }
}

通过VC++(19.16.27026.1)我尝试了几种情况来检查程序，很多情况导致编译错误：

Error C2679 binary '|': no operator found which takes a right-hand operand of type 'std::integer_sequence<uint32_t,4,5>' (or there is no acceptable conversion)

例如：

int main() {
    [[maybe_unused]] constexpr auto a = Sequence<>{} | Sequence<>{};
    [[maybe_unused]] constexpr auto b = Sequence<>{} | Sequence<3, 4, 5>{};
    [[maybe_unused]] constexpr auto c = Sequence<1>{} | Sequence<1, 2>{};
    [[maybe_unused]] constexpr auto d = Sequence<1>{} | Sequence<3, 4, 5>{}; // VC++, error C2679
}

我尝试了其他编译器，例如 GCC 8.2.0 和 Clang 7.0.0。他们编译任何案例都没有错误。

我不明白为什么会出现这个错误。有什么想法吗？

Answer 1

我明白为什么会出现这个错误。

考虑以下示例：

#include <cstdint>
#include <utility>

using UInt32 = std::uint32_t;

template<UInt32... s>
using Sequence = std::integer_sequence<UInt32, s...>;

template<UInt32... s, UInt32 x, UInt32... t>
constexpr auto foo(Sequence<s...>, Sequence<x, t...>) noexcept {
    return 0;
}

int main() {
    [[maybe_unused]] constexpr auto a = foo(Sequence<1, 2>{}, Sequence<3, 4, 5>{});
    [[maybe_unused]] constexpr auto b = foo(Sequence<1, 2, 3>{}, Sequence<4, 5>{});
}

GCC 8.2.0 和 Clang 7.0.0 compile 编译成功。但是 VC++ (VS 2017) 生成以下错误：

Error C2664: 'int func<1,2,3,4,5>(std::integer_sequence<uint32_t,1,2>,std::integer_sequence<uint32_t,3,4,5>) noexcept': cannot convert argument 1 from 'std::integer_sequence<uint32_t,1,2,3>' to 'std::integer_sequence<uint32_t,1,2>'

从错误中可以看出，由于函数模板的参数列表与模板化的重合，编译器尝试为 b 使用已经为 a 定义的函数类型，导致失败，因为最终函数参数的类型不同。另外两个编译器定义了一个新函数，新类型为 b:

a: foo<1, 2, 3, 4, 5>(Sequence<1, 2>, Sequence<3, 4, 5>)
b: foo<1, 2, 3, 4, 5>(Sequence<1, 2, 3>, Sequence<4, 5>)

于是，在计算Sequence<1>{} | Sequence<3, 4, 5>{}时出现了如下情况：

constexpr auto operator|<1, 2, 3, 4, 5>(Sequence<1, 2>, Sequence<3, 4, 5>) noexcept {
    if constexpr (find<3, 1, 2>()) { // false
        return Sequence<1, 2>{} | Sequence<3, 4, 5>{};
    } else {
        return Sequence<1, 2, 3>{} | Sequence<4, 5>{}; // error C2679
    }
}

编译器尝试使用 operator|<1, 2, 4, 5>(Sequence<1, 2>, Sequence<3, 4, 5>)。

现在知道了问题的本质，我能够重写解决方案，以便它可以被所有三个编译器编译：

using UInt32 = std::uint32_t;

template<UInt32... s>
struct Sequence {};

template<template<UInt32...> typename S, UInt32... s>
constexpr bool empty([[maybe_unused]] S<s...> a = {}) noexcept {
    return sizeof...(s) == 0;
}

template<template<UInt32...> typename S, UInt32 x, UInt32... s>
constexpr UInt32 first([[maybe_unused]] S<x, s...> a = {}) noexcept {
    return x;
}

template<template<UInt32...> typename S>
constexpr auto tail([[maybe_unused]] S<> a = {}) noexcept {
    return S<>{};
}

template<template<UInt32...> typename S, UInt32 x, UInt32... s>
constexpr auto tail([[maybe_unused]] S<x, s...> a = {}) noexcept {
    return S<s...>{};
}

template<UInt32 x, template<UInt32...> typename S>
constexpr bool find([[maybe_unused]] S<> a = {}) noexcept {
    return false;
}

template<UInt32 x, template<UInt32...> typename S, UInt32 v, UInt32... s>
constexpr bool find([[maybe_unused]] S<v, s...> a = {}) noexcept {
    if constexpr (v == x) {
        return true;
    } else {
        return find<x>(S<s...>{});
    }
}

template<UInt32... s, typename S2>
constexpr auto operator|(Sequence<s...>, S2) noexcept {
    if constexpr (sizeof...(s) == 0) {
        return S2{};
    } else if constexpr (empty(S2{})) {
        return Sequence<s...>{};
    } else {
        constexpr auto x = first(S2{});
        if constexpr (find<x>(Sequence<s...>{})) {
            return Sequence<s...>{} | tail(S2{});
        } else {
            return Sequence<s..., x>{} | tail(S2{});
        }
    }
}

VC++错误，编译时顺序操作

VC++ error, compile-time sequence operations

c++

templates

visual-c++

variadic-templates

c++17