含std::vector的std::variant引发GCC模板函数未初始化警告问题

在模板函数中遇到了无法理解的-Wmaybe-uninitialized警告，疑惑作为if判断条件的变量为何会被判定为可能未初始化。

注意：当从std::variant中移除std::vector<std::byte>类型时，该警告会消失。

简化后的代码 #

#include <cstdint>
#include <iostream>
#include <stdexcept>
#include <type_traits>
#include <variant>
#include <vector>

struct INOValue : std::variant<uint8_t, uint16_t, uint32_t, uint64_t, int64_t, std::vector<std::byte>>
{
    using variant::variant;
};

template <typename T> constexpr T ToBiggerVariable( INOValue aVariable )
{
    if constexpr( std::is_same<T, uint64_t>::value )
    {
        uint64_t lReturn = 0;
        if ( auto *lPointer = std::get_if<uint8_t>( &aVariable ) )
        {
            lReturn = *lPointer;
        }
        else if( auto *lPointer2 = std::get_if<uint16_t>( &aVariable ) )
        {
            lReturn = *lPointer2;
        }
        else if( auto *lPointer3 = std::get_if<uint32_t>( &aVariable ) )
        {
            lReturn = *lPointer3;
        }
        else if( auto *lPointer4 = std::get_if<uint64_t>( &aVariable ) )
        {
            lReturn = *lPointer4;
        }
        else
        {
            throw std::logic_error( "Invalid conversion to uint64" );
        }
        return lReturn;
    }
    else
    {
        throw std::logic_error( "Type not handled" );
    }
}

int main()
{
    INOValue lValue        = uint64_t( 12345 );
    const uint64_t lValue2 = ToBiggerVariable<uint64_t>( lValue );
    if( lValue2 > 0 )
        std::cout << "OK";
    return 0;
}

警告信息 #

在lPointer2和lPointer3处收到相同的警告：

Warning     ‘*(unsigned int*)((char*)&<unnamed> + offsetof(INOValue, INOValue::<unnamed>.std::variant<unsigned char, short unsigned int, unsigned int, long unsigned int, long int, std::vector<std::byte, std::allocator<std::byte> > >::<unnamed>.std::__detail::__variant::_Variant_base<unsigned char, short unsigned int, unsigned int, long unsigned int, long int, std::vector<std::byte, std::allocator<std::byte> > >::<unnamed>.std::__detail::__variant::_Move_assign_base<false, unsigned char, short unsigned int, unsigned int, long unsigned int, long int, std::vector<std::byte, std::allocator<std::byte> > >::<unnamed>.std::__detail::__variant::_Copy_assign_base<false, unsigned char, short unsigned int, unsigned int, long unsigned int, long int, std::vector<std::byte, std::allocator<std::byte> > >::<unnamed>.std::__detail::__variant::_Move_ctor_base<false, unsigned char, short unsigned int, unsigned int, long unsigned int, long int, std::vector<std::byte, std::allocator<std::byte> > >::<unnamed>.std::__detail::__variant::_Copy_ctor_base<false, unsigned char, short unsigned int, unsigned int, long unsigned int, long int, std::vector<std::byte, std::allocator<std::byte> > >::<unnamed>.std::__detail::__variant::_Variant_storage<false, unsigned char, short unsigned int, unsigned int, long unsigned int, long int, std::vector<std::byte, std::allocator<std::byte> > >::_M_u))’ may be used uninitialized [-Wmaybe-uninitialized]

CMakeLists配置（供参考） #

cmake_minimum_required(VERSION 3.16)

project(test_init
    LANGUAGES CXX
)

if(MSVC)
    add_compile_options(/W4)
    add_compile_options(/we4706 /we4275) #Assignement in if && dll link
else(MSVC)
    set(cxx_flags -Wsuggest-override -Werror=suggest-override)
    add_compile_options(-Wall -Wextra -Wpedantic -Wno-unknown-pragmas)
endif(MSVC)

add_executable(test_init main.cpp)

set_target_properties(test_init PROPERTIES
    CXX_STANDARD 17
    CXX_STANDARD_REQUIRED YES
    CXX_EXTENSIONS NO
)

环境信息 #

• GCC版本：11.4.0（Ubuntu 11.4.0-1ubuntu1~22.04）
• 复现环境：WSL、基于Ubuntu 22.04的Docker容器

Dockerfile内容（省略OpenCV、Qt等特定安装项） #

FROM ubuntu:22.04

RUN apt -y update && \
    apt -y upgrade && \
    apt-get -y install \
    build-essential \
    gdb \
    ninja-build \
    cmake \
    wget \
    unzip \
    git \
    python3 \
    python3-pip \
    curl \
    cppcheck \
    sudo

内容的提问来源于stack exchange，提问作者David Levy