Usually, people will learn from someone that is fluent in that language and is a native speaker, allowing them to learn from those to truly know the language. Others, though, learn from non-native speakers who also know the language quite well. There are some ups and downs of learning from an non-native speaker compared to a native speaker being the teacher.
So, what are the pros and cons of learning from an non-native speaker rather than a native speaker? Please use studies to support your answer, not opinions!