Copy Constructor DIP and implementation

[Nicholas Wilson]

On Wednesday, 19 September 2018 at 00:05:15 UTC, Jonathan M Davis 
wrote:
> On Tuesday, September 18, 2018 10:58:39 AM MDT aliak via 
> Digitalmars-d- announce wrote:
>> This will break compilation of current code that has an 
>> explicit copy constructor, and the fix is simply to add the 
>> attribute @implicit.
>
> In that case, why not just use a transitional compiler switch? 
> Why force everyone to mark their copy constructors with 
> @implicit forever? The whole point of adding the attribute was 
> to avoid breaking existing code.
>
> - Jonathan M Davis

Apparently because extending copy constructors are intended to be 
extended to types other than typeof(this), which would also be 
implicit, and then @implicit is to make sure that you actually 
want this(ref SomethingElse){ ... } to be implicitly called when

A a;
B b;
...
b = a;

which I think is a bad idea, not the least of which is because 
thats what opAssign is for.

See https://github.com/dlang/DIPs/pull/129#discussion_r218006614