Why the error message says "non-marker protocol 'Dev'" when its a Marker protocol?
protocol Dev {}
class Emp<T> {}
extension Emp: Dev where T: Sendable {}
Arent both Dev and Sendable a Marker protocol? [check screenshot]
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A marker protocol is (for now) a compiler-internal feature using the underscored attribute @_marker. For practical purposes, the diagnostics is really saying for end users that you can only conditionally conform a protocol to Sendable based on a Sendable constraint.
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Just curious, why is this the case? Additionally, if I have
struct S<T> {
var t: T
}
protocol P {
associatedtype Value: Sendable & Equatable
}
extension S: P where T: Sendable & Equatable { }
This error will prevent compilation. What's the way around it?
One of the features of marker protocols is that they exist only at compile time and not at runtime. But non-marker protocol conformances can be discovered/inspected at runtime via, e.g., as?. If the conformance of a non-marker protocol depended on conformance to a marker protocol there would be no way to resolve such runtime queries.
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