I have a basically empty QtWidgets project (Just a main window with buttons that don't do anything yet) that builds just fine, but the moment I add qcustomplot.cpp and qcustomplot.h it throws 122 errors (and would throw more but it cuts itself off) when I try to build it. The only other error is E2422 defaulted default constructor cannot be constexpr because the corresponding implicitly declared default constructor would not be constexpr on qrunnable.h, but that's there even when it does build successfully. The main culprit seems to be QCPVector2D, it says that it is not a class or namespace name repeatedly throughout the list of errors, QCPPainter also shows up with the same problem. I'm quite new to this coding thing and not really sure what I'm doing.