Conditional conformance to non-marker protocol cannot depend on conformance of 'T' to non-marker protocol 'Sendable'

[RTXGamer]

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]

[xwu (Xiaodi Wu)]

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.

[JackYoustra]

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?

[Jumhyn (Frederick Kellison-Linn)]

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.