The description of the polymorph spell states that "A shapechanger automatically succeeds on this saving throw". (Later printings state instead that "The spell has no effect on a shapechanger".)
Druids have the Wild Shape feature, and although numerous textual references refer to this ability, at no point (that I can find) are they specifically called "shapechangers".
Are druids considered "shapechangers" and thus unaffected by the polymorph spell?
Now I know that I would rule "yes, they are" in a seat-of-my-pants ruling, but is there a RAW 5e reference that makes this correlation and supports the case for druids automatically being unaffected by a polymorph spell?