std::optional - construct empty with {} or std::nullopt?

Question

I thought that initializing a std::optional with std::nullopt would be the same as default construction.

They are described as the same at cppreference, as form (1)<

Answer 1

In this case, {} invokes value-initialization. If optional's default constructor is not user-provided (where "not user-provided" means roughly "is implicitly declared or explicitly defaulted within the class definition"), that incurs zero-initialization of the entire object.

Whether it does so depends on the implementation details of that particular std::optional implementation. It looks like libstdc++'s optional's default constructor is not user-provided, but libc++'s is.