cmp/report_reflect.go
func (m visitedPointers) Visit(v reflect.Value) bool {
	p := value.PointerOf(v)
	_, visited := m[p]
	m[p] = struct{}{}
	return visited
}
func (m visitedPointers) Visit(v reflect.Value) bool {
	p := value.PointerOf(v)
	_, visited := m[p]
	m[p] = struct{}{}
	return visited
}
// Visit inserts pointer v into the visited map and reports whether it had
// already been visited before.