QT application will not compile when OpenCV 4.2 is used - c++

Question

I'm trying to build an application using OpenCV and QT, working out of QT creator. I've used this combination in the past from Visual Studio without any problems, but I have started using a Linux workstation as my main dev box, so have switched to QTCreator as an IDE.
The application receives data over the network and displays some of it as an image; all the other stuff works ok and has been tested; it's just showing the image that causes the problem.
The build error occurs when I call imshow:
Mat image = Mat(1944, 2592, CV_16U, (void*)images[imgCount].data());
imshow("disp", image);
And the build output looks like this (I have shortened some of the file paths):
g++ -c -pipe -g -std=gnu++11 -Wall -Wextra -D_REENTRANT -fPIC -DQT_DEPRECATED_WARNINGS -DQT_QML_DEBUG -DQT_NETWORK_LIB -DQT_CORE_LIB -I../RasPiRawImageReceiverTest -I. -isystem /usr/local/include/opencv4 -isystem /usr/include/qt -isystem /usr/include/qt/QtNetwork -isystem /usr/include/qt/QtCore -I. -I/usr/lib/qt/mkspecs/linux-g++ -o main.o ../RasPiRawImageReceiverTest/main.cpp
g++ -c -pipe -g -std=gnu++11 -Wall -Wextra -D_REENTRANT -fPIC -DQT_DEPRECATED_WARNINGS -DQT_QML_DEBUG -DQT_NETWORK_LIB -DQT_CORE_LIB -I../RasPiRawImageReceiverTest -I. -isystem /usr/local/include/opencv4 -isystem /usr/include/qt -isystem /usr/include/qt/QtNetwork -isystem /usr/include/qt/QtCore -I. -I/usr/lib/qt/mkspecs/linux-g++ -o receiver.o ../RasPiRawImageReceiverTest/receiver.cpp
g++ -pipe -g -std=gnu++11 -Wall -Wextra -dM -E -o moc_predefs.h /usr/lib/qt/mkspecs/features/data/dummy.cpp
/usr/bin/moc -DQT_DEPRECATED_WARNINGS -DQT_QML_DEBUG -DQT_NETWORK_LIB -DQT_CORE_LIB --include moc_predefs.h -I/usr/lib/qt/mkspecs/linux-g++ -I/QtCreatorProjects/RasPiRawImageReceiverTest -I/usr/local/include/opencv4 -I/usr/include/qt -I/usr/include/qt/QtNetwork -I/usr/include/qt/QtCore -I. -I/usr/include/c++/9.2.1 -I/usr/include/c++/9.2.1/x86_64-pc-linux-gnu -I/usr/include/c++/9.2.1/backward -I/usr/lib/gcc/x86_64-pc-linux-gnu/9.2.1/include -I/usr/local/include -I/usr/lib/gcc/x86_64-pc-linux-gnu/9.2.1/include-fixed -I/usr/include ../receiver.h -o moc_receiver.cpp
g++ -c -pipe -g -std=gnu++11 -Wall -Wextra -D_REENTRANT -fPIC -DQT_DEPRECATED_WARNINGS -DQT_QML_DEBUG -DQT_NETWORK_LIB -DQT_CORE_LIB -I../RasPiRawImageReceiverTest -I. -isystem /usr/local/include/opencv4 -isystem /usr/include/qt -isystem /usr/include/qt/QtNetwork -isystem /usr/include/qt/QtCore -I. -I/usr/lib/qt/mkspecs/linux-g++ -o moc_receiver.o moc_receiver.cpp
main.cpp: In function ‘int main(int, char**)’:
main.cpp:9:15: warning: unused variable ‘r’ [-Wunused-variable]
9 | Receiver* r = new Receiver();
| ^
g++ -o RasPiRawImageReceiverTest main.o receiver.o moc_receiver.o -L/usr/local/lib -lopencv_core -lopencv_imgproc -lopencv_highgui -lopencv_ml -lopencv_video -lopencv_features2d -lopencv_calib3d -lopencv_objdetect -lopencv_videoio -lopencv_imgcodecs -lopencv_flann /usr/lib/libQt5Network.so /usr/lib/libQt5Core.so -lpthread
/usr/bin/ld: receiver.o: in function `Receiver::dataReady()':
receiver.cpp:52: undefined reference to `imshow(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, cv::_InputArray const&)'
collect2: error: ld returned 1 exit status
make: *** [Makefile:228: RasPiRawImageReceiverTest] Error 1
12:03:02: The process "/usr/bin/make" exited with code 2.
Error while building/deploying project RasPiRawImageReceiverTest (kit: Desktop)
When executing step "Make"
OpenCV has been compiled from the 4.2.0 stable source files downloadable from the OpenCV website and using the default CMake config. QT 5.14.1 is installed from the Manjaro repos.
From what I can tell this a C++ standard mismatch, but as far as I can tell both OpenCV and the application are being compiled with C++11. The QT .pro file has the CONFIG += c++11 line in. Other OpenCV functions work; it just seems to be this from what I have tested. I know there are other warnings in the build, but these are not a problem at the moment.
Edit: .pro looks like this:
QT -= gui
QT += network
CONFIG += c++11 console
CONFIG -= app_bundle
DEFINES += QT_DEPRECATED_WARNINGS
SOURCES += \
main.cpp \
receiver.cpp
# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target
HEADERS += \
receiver.h
INCLUDEPATH += \
/usr/local/include/opencv4
/usr/local/include
LIBS += \
-L/usr/local/lib -lopencv_core -lopencv_imgproc -lopencv_highgui -lopencv_ml -lopencv_video -lopencv_features2d -lopencv_calib3d -lopencv_objdetect -lopencv_videoio -lopencv_imgcodecs -lopencv_flann