[kazinator]

What if the object is to be mutable?

[TuxSH]

You use constinit. But this means constexpr constructor (easy with 2-phase init, not too much of a problem for singleton objects), and trivial destruction

[kazinator]

Does that guarantee that the static object is constructed before the first execution of the block?

[TuxSH]

Yes, it will be constructed at compile time. Not everything can be constinit.