swift’s ‘where’ point

// A basic protocol
protocol ProtocolC: class { }

// A protocol extension basic protocol
protocol ProtocolDForSomeClassA: ProtocolC { }

// A work class's super class
class SomeClassBaseClass { }
// A work class
class SomeClassA: SomeClassBaseClass, ProtocolDForSomeClassA { }

// A tool class for control work class with protocol
class InstanceD<T: ProtocolC> {
    
    // a instance method for call other method in extension
    func callMethodD() { self.methodD() }
    // a class method for call other method in extension
    class func callClassMethodD() { self.classMethodD() }
}

// Add methods in extension
extension InstanceD {  func methodD() { print("methodD-default") } }
extension InstanceD {  class func classMethodD() { print("classMethodD-default") } }

// Add methods in extension with Where for check protocol
extension InstanceD where T: SomeClassBaseClass { func methodD() { print("methodD-someClassBaseClass") } }
extension InstanceD where T: SomeClassBaseClass { class func classMethodD() { print("classMethodD-someClassBaseClass") } }

// Direct call extension methods, result is what i wanted
InstanceD<SomeClassA>.init().methodD()      //  result:  methodD-someClassBaseClass
InstanceD<SomeClassA>.classMethodD()        //  result:  classMethodD-someClassBaseClass

// Call extension methods for instance methods, result isn't what i wanted
InstanceD<SomeClassA>.init().callMethodD()  //  result:  methodD-default
InstanceD<SomeClassA>.callClassMethodD()    //  result:  classMethodD-default

// Add methods in extension with Where for check protocol
extension InstanceD where T: SomeClassBaseClass {
    // a instance method for call other method in extension
    func callMethodD() { self.methodD() }
    // a class method for call other method in extension
    class func callClassMethodD() { self.classMethodD() }
}