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.

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.

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.