A bug in VS2019 C++20 compiler, operator <=> ?

Question

Here is the test code.

// test.cpp : 此文件包含 "main" 函数。程序执行将在此处开始并结束。
//
#include <compare>
#include <iostream>

// 2020-03-16, gxqcn 发现编译器 bug: 如写成 const auto cmp{ *--pl <=> *--pr }, 当 0 != cmp 时，指针会多移动一次！
int main()
{
    const auto test{ []( auto a, auto b )
    {
        std::cout << "before compare: a=" << a << " b=" << b << "\n";
        const auto cmp{ a++ <=> b++ };
        std::cout << "after  compare: a=" << a << " b=" << b << "\n\n";

        // 为了对比检测 std::max() 是否有类似表现，
        std::cout << "std::max( a++, b++ ) = " << std::max( a++, b++ ) << "\n";
        std::cout << "after  std::max: a=" << a << " b=" << b << "\n\n";
    }};

    test( 1, 1 );   // 测试表明：当相等时，表现与预期相符
    test( 1, 2 );   // 测试表明：当不等时，表现与预期不符

    std::cout << "-------------------------\n\n";

    int v1[]{ 1, 2, 3, 4, 5, 6 };
    int v2[]{ 1, 2, 0, 9, 0, 6 };

    auto p1{ std::cbegin( v1 ) - 1 };
    auto p2{ std::cbegin( v2 ) - 1 };

    std::cout << "compare: v1{ 1, 2, 3, 4, 5, 6 } <=> v2{ 1, 2, 0, 9, 0, 6 }\n\n";

    for ( auto index{ 0 }; sizeof( v1 ) / sizeof( v1[ 0 ] ) != index; ++index )
    {
        if ( const auto cmp{ *++p1 <=> *++p2 }; 0 != cmp )
        {
            std::cout << "v1[" << index << "] = " << v1[ index ] << "; *p1 =" << *p1 << "\n";
            std::cout << "v2[" << index << "] = " << v2[ index ] << "; *p2 =" << *p2 << "\n";

            if ( 0 < cmp )
            {
                std::cout << "\nv1 > v2";
            }
            else
            {
                std::cout << "\nv1 < v2";
            }

            if (( v1[ index ] <=> v2[ index ] ) == cmp )
            {
                std::cout << ", it's right.\n\n";
            }
            else
            {
                std::cout << " ?! it's wrong!!\n\n";
            }

            break;
        }
    }
}

This is the result from VS2019

before compare: a=1 b=1
after compare: a=2 b=2

std::max( a++, b++ ) = 2
after std::max: a=3 b=3

before compare: a=1 b=2
after compare: a=3 b=4

std::max( a++, b++ ) = 4
after std::max: a=4 b=5

---

compare: v1{ 1, 2, 3, 4, 5, 6 } <=> v2{ 1, 2, 0, 9, 0, 6 }

v1[2] = 3; *p1 =4
v2[2] = 0; *p2 =9

v1 < v2 ?! it's wrong!!

请按任意键继续. . .

This is the result from gcc-10.0

before compare: a=1 b=1
after compare: a=2 b=2

std::max( a++, b++ ) = 2
after std::max: a=3 b=3

before compare: a=1 b=2
after compare: a=2 b=3

std::max( a++, b++ ) = 3
after std::max: a=3 b=4

---

compare: v1{ 1, 2, 3, 4, 5, 6 } <=> v2{ 1, 2, 0, 9, 0, 6 }

v1[2] = 3; *p1 =3
v2[2] = 0; *p2 =0

v1 > v2, it's right.

And the same result from clang10.0

So, is it a bug in VS2019 ?

Answer 1

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Answer 2

// test.cpp : 此文件包含 "main" 函数。程序执行将在此处开始并结束。
//
#include <compare>
#include <iostream>

// 2020-03-16, gxqcn 发现编译器 bug: 如写成 const auto cmp{ *--pl <=> *--pr }, 当 0 != cmp 时，指针会多移动一次！
int main()
{
    const auto test{ []( auto a, auto b )
    {
        std::cout << "before compare: a=" << a << " b=" << b << "\n";
        const auto cmp{ a++ <=> b++ };
        std::cout << "after  compare: a=" << a << " b=" << b << "\n\n";

        // 为了对比检测 std::max() 是否有类似表现，
        std::cout << "std::max( a++, b++ ) = " << std::max( a++, b++ ) << "\n";
        std::cout << "after  std::max: a=" << a << " b=" << b << "\n\n";
    }};

    test( 1, 1 );   // 测试表明：当相等时，表现与预期相符
    test( 1, 2 );   // 测试表明：当不等时，表现与预期不符

    std::cout << "-------------------------\n\n";

    int v1[]{ 1, 2, 3, 4, 5, 6 };
    int v2[]{ 1, 2, 0, 9, 0, 6 };

    auto p1{ std::cbegin( v1 ) - 1 };
    auto p2{ std::cbegin( v2 ) - 1 };

    std::cout << "compare: v1{ 1, 2, 3, 4, 5, 6 } <=> v2{ 1, 2, 0, 9, 0, 6 }\n\n";

    for ( auto index{ 0 }; sizeof( v1 ) / sizeof( v1[ 0 ] ) != index; ++index )
    {
        if ( const auto cmp{ *++p1 <=> *++p2 }; 0 != cmp )
        {
            std::cout << "v1[" << index << "] = " << v1[ index ] << "; *p1 =" << *p1 << "\n";
            std::cout << "v2[" << index << "] = " << v2[ index ] << "; *p2 =" << *p2 << "\n";

            if ( 0 < cmp )
            {
                std::cout << "\nv1 > v2";
            }
            else
            {
                std::cout << "\nv1 < v2";
            }

            if (( v1[ index ] <=> v2[ index ] ) == cmp )
            {
                std::cout << ", it's right.\n\n";
            }
            else
            {
                std::cout << " ?! it's wrong!!\n\n";
            }

            break;
        }
    }
}