It was discussed in Oulu, pardon for the failure to convey the
feedback all the way to the proposal author.
Some polls:
Do we think we want a mechanism for forward-declaring the public
interface of a class? 0 | 1 | 11 | 5 | 0
Encourage more work on P0309r0? 0 | 0 | 3 | 6 | 7
Some quotes from the discussion:
"This actually has the performance cost that PImpl has in that you
have to have an inline function definition for some methods (e.g.,
isEmpty) if you don't want a performance cost. And nested classes.
This doesn't work. "
"Changing the head of the class has a very high bar. "
In other words, Evolution was unconvinced by the proposal.