HashSet missing most methods

I’m trying to use HashSet but whenever I try to use any of the built in methods besides Add or Remove, I’m met with an error:

Type `HashSet<ChunkType>' does not contain a definition for `UnionWith' and no extension method `UnionWith' of type `HashSet' could be found. Are you missing an assembly reference?

I didn’t forget to include System.Collections.Generic and the compiler/IDE (Visual Studio) still recognizes the HashSet type. I’m having issues with UnionWith, IntersectWith and RemoveWhere specifically, but none of the other methods are working either.

Solved by deleting the .meta files from my scripts folder.