Thanks again for all the feedback.

Here's a draft proposal: https://cppproposals.aryan.app/flatbool.html

Please let me know what you think!

On Mon, May 25, 2026 at 2:44 PM Aryan Naraghi <aryan.naraghi_at_[hidden]>
wrote:

> Thanks for the feedback, everyone.
>
> With the exception of std::flat_*, the other container adapters (stack,
> queue, and priority_queue) define their value_type, reference, and
> const_reference in terms of the underlying container, so they just work
> with vector<bool>. See example:
> https://eel.is/c++draft/priority.queue#priqueue.overview-1
>
> I like Arthur's proposal to do the same for std::flat_*. I'll start a
> draft.
>
> On Mon, May 25, 2026 at 7:53 AM Yongwei Wu via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
>> On Mon, 25 May 2026 at 12:08, Hewill Kang via Std-Proposals <
>> std-proposals_at_[hidden]> wrote:
>>
>>>
>>>> FWIW, I found it super easy to implement a proper `flat_map` that Just
>>>> Works for all types (even bool). I've been "shipping" it in my P1144 fork
>>>> of libc++ since at least January 2023, since before Hui Xie's flat
>>>> containers were merged to Clang trunk.
>>>> https://godbolt.org/z/j1az7vY87
>>>> These days the patch is reduced to just a few lines on top of Hui's
>>>> implementation:
>>>>
>>>> https://github.com/llvm/llvm-project/commit/a3cb6f367fe50ac40cefc6bced487e141f6cbe23
>>>>
>>>> I certainly think that all vendors (yes, even libc++!) should ship an
>>>> implementation of `flat_set` and `flat_map` that Just Works, even with
>>>> `bool`. I think libc++'s current behavior is deplorable.
>>>>
>>>
>> I think libc++'s behaviour is OK, given the current wording of the
>> standard.
>>
>>
>>> Does that mean we need to change the wording of the paper standard?
>>>> Well, IMHO, vendors *should* know to Do The Right Thing regardless of
>>>> whether the paper standard requires it. But changing the paper standard
>>>> would probably help in practice, so sure, I'd encourage you to go for it.
>>>>
>>>
>> Do the right thing, if the right thing does not go against the standard.
>> Otherwise we should change the standard first.
>>
>> The trick with the paper standard is that `vector<bool>` isn't a
>>>> "sequence container"; so either you'll have to figure out how to stretch
>>>> the definition of "sequence container" to include `vector<bool>`, or you'll
>>>> have to extend the definition of `flat_set` to include `vector<bool>` as a
>>>> special case. The former would be more generally useful in the future, but
>>>> the latter is way easier.
>>>>
>>>
>> I think Aryan's approach is good and simple. Also, I won't worry about
>> `flat_set<bool>`. Only `flat_map<Key, bool>` is likely, though, honestly
>> speaking, it can be replaced with `flat_set<Key>` too...
>> --
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>>
>