Rationale (August, 2011):

Possible resolutions to this issue need to be considered in a wider context, so it is more appropriate for the Evolution Working Group.

[Adopted at the November, 2017 meeting as part of paper P0641R2.]

The current requirements of 9.5.2 [dcl.fct.def.default] paragraph 1 state that a defaulted copy constructor or copy assignment operator can have a reference to const parameter only if all its subobjects have corresponding functions with a reference to const parameter, even if that function is never called. This prevents some useful template classes.

(See also library issue 2068.)

See also issue 1426.