Additionally, an automatically declared copy constructor or copy assignment operator is defined as deleted if the class has a user-provided move constructor or move assignment operator.

What is implicitly deleted copy constructor?

Deleted implicitly-declared copy constructor

T is a union-like class and has a variant member with non-trivial copy constructor; ... T has a user-defined move constructor or move assignment operator (this condition only causes the implicitly-declared, not the defaulted, copy constructor to be deleted).

Why is copy constructor deleted?

Copy constructor (and assignment) should be defined when ever the implicitly generated one violates any class invariant. It should be defined as deleted when it cannot be written in a way that wouldn't have undesirable or surprising behaviour.