Re: float is constructible from double, C++23's std::float13_t is not

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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On Fri, 24 Mar 2023, 17:05 Filippo Bistaffa via Gcc-help, <
gcc-help@gcc.gnu.org> wrote:

> I was playing around with gcc-trunk's support for C++23's std::float13_t
> and I found out that, while float is constructible from double, as far as I
> can tell std::float13_t is not.
> In other words, the following code compiles OK:
>
> #include <stdfloat>
> #include <vector>
> int main() {
>     std::vector<double> x(5);
>     std::vector<float> y(std::begin(x), std::end(x));
> }
>
> whereas the following does not:
>
> #include <stdfloat>
> #include <vector>
> int main() {
>     std::vector<double> x(5);
>     std::vector<std::float16_t> y(std::begin(x), std::end(x));
> }
>
> See this snippet <https://godbolt.org/z/zq7KdhY44>.
> Am I missing something or is it supposed to be like that?
>

Yes, this is the correct behaviour. The C++23 standard says:

"A prvalue of floating-point type can be converted to a prvalue of another
floating-point type with a greater or equal conversion rank ([conv.rank]
<https://eel.is/c++draft/conv.rank>).
A prvalue of standard floating-point type can be converted to a prvalue of
another standard floating-point type."