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

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.

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

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

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

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

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

_Generic and typeof are not valid C++.

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

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++.

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

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[diff.expr] Undocumented change of `0 == 0` type from `int` to `bool` #9299

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[diff.expr] Undocumented change of 0 == 0 type from int to bool #9299

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frederick-vs-ja commented on Sep 3, 2026

jensmaurer commented on Sep 3, 2026

eisenwave commented on Sep 3, 2026

jensmaurer commented on Sep 3, 2026

frederick-vs-ja commented on Sep 4, 2026

Halalaluyafail3 commented on Sep 4, 2026

Halalaluyafail3 commented on Sep 4, 2026

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[diff.expr] Undocumented change of `0 == 0` type from `int` to `bool` #9299