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Re: A counterintuitive breaking change related to new comparisons

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On 1/4/2022 12:56 AM, Manfred wrote:
> On 1/3/2022 11:24 PM, Andrey Tarasevich wrote:
>> On 1/3/2022 9:48 AM, Andrey Tarasevich wrote:
>>> A colleague discovered that switching from `-stc=c++17` to 
>>> `-std=c++20` in their project resulted in a different behavior from 
>>> some associative containers. A bit of research allowed to narrow down 
>>> the culprit to what can be demonstrated by the following minimalist 
>>> example
>>>
>>>    #include <iostream>
>>>
>>>    struct S
>>>    {
>>>      char c;
>>>
>>>      operator const char *() const { return &c; }
>>>
>>>      friend bool operator <(const S& lhs, const S& rhs)
>>>        { return lhs.c < rhs.c; }
>>>    };
>>>
>>>    int main()
>>>    {
>>>      std::pair<int, S> p1{}, p2{};
>>>      std::cout << (p1 < p2) << (p2 < p1) << std::endl;
>>>    }
>>>
>>> C++17 compilers output `00`. C++20 compilers output `01` (or, 
>>> perhaps, `10`).
>>>
>>> The obvious guess is that comparisons for `std::pair` work 
>>> differently in C++20 after the introduction of `<=>`. And indeed, the 
>>> `<=>` operator for `std::pair` in this case ignores the user-defined 
>>> `<` and instead opts for raw pointer comparison through conversion to 
>>> `const char *`. This is a rather surprising and counterintuitive 
>>> breaking change, to put it mildly...
>>>
>>> If we remove the user-defined conversion to `const char *`, C++20 
>>> will use the user-defined `<` and also output `00`.
>>>
>>> The above results were obtained with GCC. Clang 10 outputs `00` even 
>>> in `-std=c++20` mode, but Clang 11 and later output `01`.
>>
>> To take out of the picture the irrelevant matter of undefined pointer 
>> comparison, here's another example
>>
>>    #include <iostream>
>>
>>    struct S
>>    {
>>      int a;
>>
>>      S(int a) : a(a) {}
>>
>>      operator int() const { return a; }
>>
>>      friend bool operator <(const S& lhs, const S& rhs)
>>        { return lhs.a > rhs.a; }
>>    };
>>
>>    int main()
>>    {
>>      std::pair<int, S> p1{ 0, 1 }, p2{ 0, 2 };
>>      std::cout << (p1 < p2) << (p2 < p1) << std::endl;
>>    }
>>
>> This program outputs `01` in C++17 mode (and earlier)
>> http://coliru.stacked-crooked.com/a/98f0b6b57e9caf45
>>
>> but outputs `10` in C++20 mode
>> http://coliru.stacked-crooked.com/a/3f8c3e96cd39f596
>>
>> In C++17 the user-defined comparison operator is used. In C++20 
>> conversion to `int` and subsequent comparison of `int`s is used.
>>
> 
> That's really odd indeed.
> Are you sure that is is the C++20 standard that mandates this behavior?
> I wouldn't be surprised if this were a plain bug in the implementation - 
> possibly related with the changes that you mention above - but still a bug.
> 
> In fact, gcc 11.2 with plain -std=c++20 gives '01':
> 
> https://www.godbolt.org/z/rEjfehdnY

scratch that last bit (for some reason godbolt uses diferent comiler 
selections fro compilation and program output)

https://www.godbolt.org/z/be1q7PErE

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A counterintuitive breaking change related to new comparisons Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-03 09:48 -0800
  Re: A counterintuitive breaking change related to new comparisons Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2022-01-03 21:06 +0000
    Re: A counterintuitive breaking change related to new comparisons Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-03 14:13 -0800
      Re: A counterintuitive breaking change related to new comparisons Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2022-01-04 00:23 +0000
        Re: A counterintuitive breaking change related to new comparisons Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-03 16:33 -0800
          Re: A counterintuitive breaking change related to new comparisons Juha Nieminen <nospam@thanks.invalid> - 2022-01-04 08:27 +0000
            Re: A counterintuitive breaking change related to new comparisons "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-01-04 17:24 +0100
            Re: A counterintuitive breaking change related to new comparisons Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-04 10:26 -0800
        Re: A counterintuitive breaking change related to new comparisons Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2022-01-04 00:39 +0000
  Re: A counterintuitive breaking change related to new comparisons Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-03 14:24 -0800
    Re: A counterintuitive breaking change related to new comparisons "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-01-03 23:55 +0100
    Re: A counterintuitive breaking change related to new comparisons Manfred <noname@add.invalid> - 2022-01-04 00:56 +0100
      Re: A counterintuitive breaking change related to new comparisons Manfred <noname@add.invalid> - 2022-01-04 00:59 +0100
      Re: A counterintuitive breaking change related to new comparisons Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-03 16:59 -0800
        Re: A counterintuitive breaking change related to new comparisons Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2022-01-04 01:27 +0000
        Re: A counterintuitive breaking change related to new comparisons Öö Tiib <ootiib@hot.ee> - 2022-01-04 14:49 -0800
          Re: A counterintuitive breaking change related to new comparisons Öö Tiib <ootiib@hot.ee> - 2022-01-04 15:16 -0800
          Re: A counterintuitive breaking change related to new comparisons Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-05 09:06 -0800
  Re: A counterintuitive breaking change related to new comparisons "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-01-03 18:51 -0800
  Re: A counterintuitive breaking change related to new comparisons wij <wyniijj@gmail.com> - 2022-01-04 03:53 -0800
    Re: A counterintuitive breaking change related to new comparisons wij <wyniijj@gmail.com> - 2022-01-04 21:59 -0800

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