On 02/09/2025 14:24, Hans Åberg via Std-Proposals wrote:
>
>
>> On 2 Sep 2025, at 14:14, Jan Schultke <janschultke_at_[hidden]> wrote:
>>
>> You seem to be confusing some mostly unrelated concepts.
>>
>>>> 1. C does not allow _BitInt(1); should C++ to make generic programming
>>>> more comfortable?
>>>
>>> The ring ℤ/2ℤ of integers modulo 2, also a field, is isomorphic to the Boolean ring 𝔹 having exclusive or as addition and logical conjunction as multiplication.
>>>
>>> If bool 1+1 is defined to 0, then it is already in C++.
>>
>> Whether there is some other C++ thing that works mathematically the
>> same doesn't say anything about whether _BitInt(1) is valid or should
>> be valid. The issue is regarding a specific type.
>
> It could have been defined to be the same as bool.

No, it could not. _BitInt(1), if it is to exist, has the two values -1
and 0. Like all signed integer types, arithmetic overflow on it is
undefined, and like all _BitInt types, there is no integer promotion.
Thus for _BitInt(1), (-1) + (-1) is UB.

unsigned _BitInt(1) can hold 0 and 1, has no integer promotion, and has
modulo arithmetic behaviour - thus 1 + 1 gives 0 for "unsigned _BitInt(1)".

Both signed _BitInt(1) and unsigned _BitInt(1) are integer types
representing numbers. In C23 (and presumably C++ when it gets _BitInt),
these are included in the "standard integer types". "bool", on the
other hand, is /not/ a standard integer type in C++ (though it is one in C).

Being able to represent two distinct values does not mean a type behaves
like "bool".

>
>>> However, in GCC and Clang, bool 1+1 = 1, and I could not see what the standard specifies.
>>
>> Any nonzero integer is true when converted to bool, so "bool(true +
>> true)" is essentially "1 + 1 != 0". bool gets promoted to int before
>> any operation.
>
> Apparently, there is no operator+ for bool. I did:
> bool a = 1, b = 1, c = a + b;
>

Correct. There is also no "+" operator for char, or short int - other
than _BitInt, types smaller than "int" get promoted to "int" before the
operation.