I ran into this problem recently by converting Obj-C to a hybrid with Swift / Obj-C. I was fully aware of the "m" / "mm" problem, so that was not my problem.
What caused this problem for me was that I included the include file in the 'bridging' header, which referred to OpenCV - both the #include file and some definitions. Oops
Answer: check how your #include link file somehow refers to opencv - remove.
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