On Sat, 25 Mar 2023, 11:29 Filippo Bistaffa, <filippo.bistaffa@gmail.com>
wrote:

> I'm not sure I fully understand that quote: double can be converted to
> float (even if float has a lower rank) because they are both "standard",
> whereas the same doesn't apply to std::float16_t?
>

Correct. float16_t is an extended floating-point type.

https://en.cppreference.com/w/cpp/language/types#Floating-point_types


> On Fri, Mar 24, 2023, 21:22 Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>
>>
>>
>> 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."
>>
>>
>>
>>