I note the Icelandic composer Jón Leifs is here listed as one might expect, Leifs, Jón. However, both the Library of Congress and Wikipedia explain that In Iceland what we might call the surname is really a patronymic, so the true name is Jón [son of] Leifs. For this reason both of those organizations alphabetize Icelandic names under what we call the forename—in this case, Jón.
Might IMSLP do the same?
Jón Leifs and other Icelandic names
Moderator: kcleung