c++ - std::unique_ptr structure member to the structure type -

is

struct {     std::unique_ptr<a> a; }; 

allowed standard? don't think container types std::set there special unique_ptr?

yes, it's explicitly allowed. c++14 (n4140) 20.8.1/5:

... template parameter t of unique_ptr may incomplete type.

it allowed std::shared_ptr , std::weak_ptr, using similar wording.