consteval上下文无法更新std::variant的编译问题求助

我正在编写C++编译期解析器，使用Microsoft Visual Studio Community 2019（版本17.8.3）时遇到编译问题。以下代码在consteval函数中更新std::variant时触发编译错误，但将函数签名改为constexpr即可正常编译。

问题代码 #

#include <array>
#include <variant>

using namespace std;

enum class TokenErrors
{
    UNINITIALISED,
    TK_ERROR_SYMBOL,
    TK_ERROR_PATTERN,
    TK_EOF
};

enum class MyTokenType
{
    TK_SYMBOL
};

template <typename TokenType, int num_states>
struct Lexer
{
    std::array<TokenType, num_states> final_states;
};

static consteval auto foo()
{
    auto lxr = Lexer<std::variant<MyTokenType, TokenErrors>, 5>{};
    for (auto &x: lxr.final_states)
        x = TokenErrors::UNINITIALISED;
    return lxr;
}

int main()
{
    auto res = foo();
}

编译错误信息 #

Build started at 17:22...
1>------ Build started: Project: EfficientCompiler, Configuration: Debug x64 ------
1>Scanning sources for module dependencies...
1>main.cpp
1>C:\Users\Aakash\source\repos\EfficientCompiler\EfficientCompiler\main.cpp(35,19): error C2440: '<function-style-cast>': cannot convert from 'initializer list' to 'Lexer<std::variant<MyTokenType,TokenErrors>,5>'
1>C:\Users\Aakash\source\repos\EfficientCompiler\EfficientCompiler\main.cpp(35,19):
1>Invalid aggregate initialization
1>  C:\Users\Aakash\source\repos\EfficientCompiler\EfficientCompiler\main.cpp(35,19):
1>  'initializing': cannot convert from 'initializer list' to 'MyTokenType'
1>      C:\Users\Aakash\source\repos\EfficientCompiler\EfficientCompiler\main.cpp(35,19):
1>      Too many braces around initializer for 'MyTokenType'
1>  C:\Users\Aakash\source\repos\EfficientCompiler\EfficientCompiler\main.cpp(35,19):
1>  'initializing': cannot convert from 'initializer list' to 'MyTokenType'
1>      C:\Users\Aakash\source\repos\EfficientCompiler\EfficientCompiler\main.cpp(35,19):
1>      Too many braces around initializer for 'MyTokenType'
1>  C:\Users\Aakash\source\repos\EfficientCompiler\EfficientCompiler\main.cpp(35,19):
1>  'initializing': cannot convert from 'initializer list' to 'MyTokenType'
1>      C:\Users\Aakash\source\repos\EfficientCompiler\EfficientCompiler\main.cpp(35,19):
1>      Too many braces around initializer for 'MyTokenType'
1>  C:\Users\Aakash\source\repos\EfficientCompiler\EfficientCompiler\main.cpp(35,19):
1>  'initializing': cannot convert from 'initializer list' to 'MyTokenType'
1>      C:\Users\Aakash\source\repos\EfficientCompiler\EfficientCompiler\main.cpp(35,19):
1>      Too many braces around initializer for 'MyTokenType'
1>  C:\Users\Aakash\source\repos\EfficientCompiler\EfficientCompiler\main.cpp(35,19):
1>  'initializing': cannot convert from 'initializer list' to 'MyTokenType'
1>      C:\Users\Aakash\source\repos\EfficientCompiler\EfficientCompiler\main.cpp(35,19):
1>      Too many braces around initializer for 'MyTokenType'
1>C:\Users\Aakash\source\repos\EfficientCompiler\EfficientCompiler\main.cpp(35,16): error C7595: 'foo': call to immediate function is not a constant expression
1>Done building project "EfficientCompiler.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========
========== Build completed at 17:22 and took 01.116 seconds ==========

这个问题源于VS2019对C++20 consteval特性的实现缺陷，在处理std::variant的默认初始化后赋值时存在兼容性问题。可以通过以下几种方式解决：

• 显式构造std::variant赋值：修改循环中的赋值语句，显式构造std::variant对象，避免隐式转换的问题：

static consteval auto foo()
{
    auto lxr = Lexer<std::variant<MyTokenType, TokenErrors>, 5>{};
    for (auto &x: lxr.final_states)
        x = std::variant<MyTokenType, TokenErrors>{TokenErrors::UNINITIALISED};
    return lxr;
}

• 直接初始化数组：在创建Lexer对象时，直接初始化final_states数组，跳过后续赋值步骤，完全在编译期完成初始化：

static consteval auto foo()
{
    return Lexer<std::variant<MyTokenType, TokenErrors>, 5>{
        .final_states = {
            std::variant<MyTokenType, TokenErrors>{TokenErrors::UNINITIALISED},
            std::variant<MyTokenType, TokenErrors>{TokenErrors::UNINITIALISED},
            std::variant<MyTokenType, TokenErrors>{TokenErrors::UNINITIALISED},
            std::variant<MyTokenType, TokenErrors>{TokenErrors::UNINITIALISED},
            std::variant<MyTokenType, TokenErrors>{TokenErrors::UNINITIALISED}
        }
    };
}

• 升级编译器版本：考虑升级到Visual Studio 2022，新版本对C++20特性的支持更完善，已经修复了这类consteval下的std::variant处理问题。

内容的提问来源于stack exchange，提问作者Aakash Gupta