How can we construct a std::pmr::string using value_to, with user-provided std::pmr::memory_resource*?
Here, the object and the json value are using different memory_resource sources.

The customization function's signature is T tag_invoke( const value_to_tag< T >&, const value& ), which does not take any Allocator as parameter. So there is no way to provide the allocator.

It seems we can only construct std::pmr::string from std::pmr::get_default_resource()?