[swift-evolution] [Planning][Request] "constexpr" for Swift 5

[Tino Heth]

> If you read my reply to Daniel Vollmer, you’ll find that we’re thinking about the exact same thing. 
Good to hear — read that post later, but I guess repeating doesn't hurt here: People still talk about macros... ;-)

>> So while I think they’re both great features to have, I assume the core team will have higher priorities on their schedule, as I don’t see how these features wouldn’t eat up most resources for at least (!) one Swift release-cycle.
> 
> That’s just it: the core team is already prioritizing a lot of features that would greatly benefit from this. In fact, if this would be implemented, a large portion of the swift compiler could be moved into the standard library, because it wouldn’t be magic any more. This would single-handedly reduce the implementation time and effort of most proposals by a very large margin.

… and I like specific examples ;-), so out of my head, four features that people want to have and that could be added easily with "Meta-Swift":
- Forwarding of protocol conformance (Kotlin, for example, has this: When a member conforms to a protocol, you don't have to write a bunch of methods that just say "let my member do this")
- init with reduced boilerplate
- Subtyping for non-class types, including a "newtype" option
- Property behaviours

Imho the reflection capabilities that are needed to make the feature fly are the big tasks — using this information shouldn't be that hard, as it's just a explicit way of telling the compiler what to do.
Assuming that reflection will be improved anyways, it might be a very cheap feature with huge benefit.
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