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

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.