5 Commits

Author	SHA1	Message	Date
Doug Gregor	618378ae48	Ban use of marker protocols in conditional conformances to non-marker protocols ... These conditional conformances could not be properly evaluated at runtime, so ban them. We still allow conditional conformances to marker protocols.	2021-02-20 22:04:22 -08:00
Doug Gregor	9df17fff93	Allow implied conditional conformances to marker protocols. ... This improves source compatibility in cases where a marker protocol is introduced after-the-fact (e.g., for `Error`), and is safe because there's no runtime component to marker protocols.	2021-02-20 21:10:18 -08:00
Doug Gregor	66cd7d82ae	Marker protocols are self-conforming.	2021-02-18 23:44:39 -08:00
Doug Gregor	85611dbf25	Allow marker protocols as existentials (but still ban casts) ... There is no problem with having an existential value that involves a marker protocol. However, since there is no runtime checking for marker protocols, ban "as?" and "is" casts to them.	2021-02-18 23:44:39 -08:00
Doug Gregor	c9d66f0d35	Add "marker" protocols, indicated by @_marker. ... A marker protocol is a protocol with no requirements and with no ABI footprint. They can be used to indicate semantics, only, and one can never have values of a marker protocol type. We still mangle marker protocols when they are used as generic requirements, because they are a distinguishing characteristic for overloading, but "no ABI footprint" means no protocol descriptors, conformance descriptors, or dynamic discovery of any kind.	2021-02-02 21:53:26 -08:00