On Monday, September 24, 2018 3:20:28 PM MDT Manu via Digitalmars-d-announce wrote: > copy-ctor is good, @implicit is also good... we want both. Even though > copy-ctor is not strictly dependent on @implicit, allowing it will > satisfy that there's not a breaking change, it it will also > self-justify expansion of @implicit as intended without a separate and > time-consuming fight, which is actually the true value of this DIP!
@implicit on copy constructors is outright bad. It would just be a source of bugs. Every time that someone forgets to use it (which plenty of programmers will forget, just like they forget to use @safe, pure, nothrow, etc.), they're going to have a bug in their program. However, unlike, with attributes like @safe or pure, they're not going to get a compiler error in their program; they're going to get a logic error. They may or may not find that bug quickly, but the compiler isn't going to point it out to them. And if @implicit weren't a thing, then the problem wouldn't even exist. @implicit is trying to get around breaking an extremely unlikely piece of code which would be far better fixed by using a transitional compiler flag and which adds _no_ value for copy constructors in the long run. Even if we do later add @implicit to the language for regular constructors, it has no business on copy constructors. And its value on other constructors needs to be evaluated and examined on its own separately from the issue of copy constructors. We should not be letting @implicit be put onto copy constructors just to get it into the language. If it's such a valuable feature on regular constructors, it should be able to get in on a separate DIP specifically for that purpose without piggybacking in on the copy constructor DIP - especially when it's clearly going to be a source of bugs if it's required to be on copy constructors. And it just needlessly adds to the attribute soup. At least if it were added for regular constructors, it would be adding value. For copy constructors, it's just adding annoyance and the risk of bugs. - Jonathan M Davis
