A template with an object-modeling API similar to std::optional,
with a value(), typedef value_type, operator* operator->.
Init examples from the unit test:
x is convertible to an Indestructible&, and can sometimes be passed directly.
We can also make the cast without naming the type with *x and x.value().
Immortal forwards (via placement new) its constructor arguments to T(args...). This performs a placement new onto a std::aligned_buffer_t, so there is no allocation as there would be with the traditional approach, and there is no destructor call.
Having a common idiom and place to hang documentation will be useful in cutting down on the very common misuses and bugs we have regarding init order and shutdown order.
Importantly, it becomes a "greppable" and auditable best practice to identify and contain such objects, which are often suspects in startup and shutdown problems, or need to be called out in refactorings to reduce shared global state.