On Tuesday 3 September 2024 00:55:45 CEST Sebastian Wittmeier via Std-
Proposals wrote:
> In general a container can do
>
> if constexpr (std::is_same_v<T, int>)
> ...
>
> Is this true, if the underlying type is int and T is a subtype?

A concrete but very ill-advised example:

std::optional<T> where T is a floating-point type can decide it only stores
normalised results and use the negative zero representation as the marker for
the disengaged state. That would make sizeof(std::optional<T>) == sizeof(T).

But if T is a struct wrapping a floating-point value and is thus itself not
std::is_floating_point_v<T>, std::optional would use a separate representation.
That means the two std::optional are not isomorphic.

-- 
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
  Principal Engineer - Intel DCAI Platform & System Engineering