type Constraint interface {
	copyTo(*ConstraintMsg)
}

type ConstraintMsg struct{}

func (m *ConstraintMsg) asUnpairedVersion() UnpairedVersion {
	return nil
}

type Version interface {
	Constraint
}

type UnpairedVersion interface {
	Version
}

var _ Constraint = UnpairedVersion(nil)


// derived test case from issue #21804

type (
	_ interface{ m(B1) }
	A1 interface{ a(D1) }

type Constraint interface {
	copyTo(*ConstraintMsg)
}

type ConstraintMsg struct{}

func (m *ConstraintMsg) asUnpairedVersion() UnpairedVersion {
	return nil
}

type Version interface {
	Constraint
}

type UnpairedVersion interface {
	Version
}

var _ Constraint = UnpairedVersion(nil)


// derived test case from issue #21804

type (
	_ interface{ m(B1) }
	A1 interface{ a(D1) }