lambda는 기본적으로 생성자가 지워져있다.
link: c++
Understanding how Lambda closure type has deleted default constructor
https://en.cppreference.com/w/cpp/language/lambda 에 따르면...
Closure types are not DefaultConstructible. Closure types have no default constructor. 1
클로저 타입은 기본 생성자가 "없"습니다.
If no captures are specified, the closure type has a defaulted default constructor. Otherwise, it has no default constructor (this includes the case when there is a capture-default, even if it does not actually capture anything). 2
클로저 타입에 캡쳐가 하나도 없다면, 기본 생성자를 가질 수 있습니다.