[diff.expr] Undocumented change of `0 == 0` type from `int` to `bool`

[eisenwave]

We document in [diff.iso] that the types of string literals and character literals are changed, but we dubiously do not document that 0 == 0 (generally, equality and relatioal comparisons) produce int in C but bool in C++.

This feels like a pretty relevant difference, although it's hard to observe in practice because integer promotion gets rid of boolness in lots of cases.

[frederick-vs-ja]

Built-in && and || operators also produce int in C but bool in C++.

[jensmaurer]

Do you have an example where this is observable with C-only syntax?

[eisenwave]

static_assert(sizeof(0 == 0) == sizeof(bool));

is almost certainly failing in C, but guaranteed to be pass in C++.

And obviously, you can observe it with _Generic and typeof, but that's probably less interesting for a [diff] entry.

[jensmaurer]

_Generic and typeof are not valid C++.

[frederick-vs-ja]

We can observe these with auto and pointer variables. Although it seems quite hard to do so with pre-C23 constructs.

[Halalaluyafail3]

Here is another way that it can be observed in C23 and C++26:

#include<stdckdint.h>
int main(){
    int x;
    ckd_add(&x,0==0,0);
}

ckd_add does not accept bools, so it is invalid only in C++.

[Halalaluyafail3]

It may also be worth mentioning that some library functions such as isnormal have different return types in C++, changing from int to bool.