digitalmars.D - implicit conversion to mutable via alias this
- Nils =?UTF-8?B?Qm/Dn3VuZw==?= (34/34) Jun 16 2014 Last week I filed issues 12883  and 12884 , and made dmd pull requ...
- Nils =?UTF-8?B?Qm/Dn3VuZw==?= (1/1) Jun 17 2014 Anyone?
- Nils =?UTF-8?B?Qm/Dn3VuZw==?= (1/1) Jun 19 2014 bump
- Nils =?UTF-8?B?Qm/Dn3VuZw==?= (1/1) Jun 21 2014 I'd still appreciate feedback on this.
- "Marc =?UTF-8?B?U2Now7x0eiI=?= <schuetzm gmx.net> (8/8) Jun 21 2014 I can only guess Kenji's reasons, but I believe it's related to
- Nils =?UTF-8?B?Qm/Dn3VuZw==?= (2/6) Jun 21 2014 Thanks a bunch. That sheds quite some light on it.
Last week I filed issues 12883  and 12884 , and made dmd pull request #3654  to fix them. The changes would make it possible to use alias this for implicit conversion to mutable when the conversion is otherwise not possible. Issue 12883 is about structs. dmd's current rules for conversion of structs are essentially: 1) If the unqualified source and target types match, and the members can be converted, do that. 2) Otherwise, if the unqualified types do not match, try alias this. 3) Otherwise, fail. I'd like to change step 2 to 2) Otherwise, try alias this. That is, in particular, if the unqualified types match but the members can't be converted, try alias this. Kenji rejected the pull request saying that conversion to mutable should be handled by postblit. As far as I can see, that doesn't work right now. And I don't see how it's supposed to work. We can't allow conversion to mutable just because there's a postblit. And there is no way to mark/overload the postblit for a const-stripping conversion, is there? Kenji, could you elaborate on how postblit is supposed to take care of this? Or maybe someone else has some insight? Or maybe you think that PR #3654 is totally awesome and should be pulled? ;) As for classes (issue 12884), dmd already accepts alias this to mutable here. But one way or the other it's buggy. The motivation for all this came from issue 12885 : There is a hack in place in dmd to allow implicit conversion of const(Rebindable!(const Object)) to mutable. In my opinion it's too permissive and lets code be accepted that should be rejected. With to- mutable-via-alias-this the conversion could be implemented in Rebindable itself and the hack could be reverted.  https://issues.dlang.org/show_bug.cgi?id=12883  https://issues.dlang.org/show_bug.cgi?id=12884  https://github.com/D-Programming-Language/dmd/pull/3654  https://issues.dlang.org/show_bug.cgi?id=12885
Jun 16 2014
Jun 17 2014
Jun 19 2014
I'd still appreciate feedback on this.
Jun 21 2014
"Marc =?UTF-8?B?U2Now7x0eiI=?= <schuetzm gmx.net> writes:
I can only guess Kenji's reasons, but I believe it's related to DIP49, which he created: http://wiki.dlang.org/DIP49 The DIP describes how copying between differently qualified types should work. I guess his stance is that this should always go via the postblit, probably to avoid uncertainties about postblit or alias this has priority over the other. IMO this makes sense. (And you're right, this is currently not implemented.)
Jun 21 2014