As mentioned in the comments, I had to do two things:
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changing Compilation Mode to Whole Module inside Project settings/Build Settings:
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reordering the files under Project settings/Build Phases/Compile Sources. Specifically, I brought the files that had an error to the front of the list.
Protip: if you search for the name of the file and there is more than one result, dragging the file to the top in that smaller list will still bring it to the front.