@_nonoverride に関しては
public protocol BaseP { func foo() func bar() } public extension BaseP { func foo() { print("BaseP.foo()") } func bar() { print("BaseP.bar()") } } public protocol DerivedP: BaseP { override func foo() @_nonoverride func bar() } public extension DerivedP { func foo() { print("DerivedP.foo()") } func bar() { print("DerivedP.bar()") } } public struct Concrete: DerivedP {}