A defaulted prospective destructor ([class.dtor]
that is not a destructor is defined as deleted.
A defaulted special member function
that is neither a prospective destructor nor
an eligible special member function ([special]
is defined as deleted.
A function is
if it is user-declared and not explicitly
defaulted or deleted on its first declaration.
A user-provided explicitly-defaulted function
(i.e., explicitly defaulted after its first declaration)
is defined at the point where it is explicitly defaulted; if such a function is implicitly
defined as deleted, the program is ill-formed.
Declaring a function as defaulted after its first declaration can provide
efficient execution and concise
definition while enabling a stable binary interface to an evolving code
— end note