In libpyside/CMakeLists.txt we have a hacky piece of code that assumes that if qtcore private headers are available, then qml / quick private headers should also be available, and simply rewrites "QtCore" to "QtQml" in the paths, without checking if the private headers actually exist.
This causes a build failure on Debian Testing where all private include headers are separated into different packages, and thus it is possible to have qtcore private headers installed, but not qml ones.
Build errors look like
from /home/ale/tmp/pyside-setup/testenv3d_build/py3.5-qt5.7.1-64bit-debug/pyside2/libpyside/signalmanager.cpp:65:/usr/include/x86_64-linux-gnu/qt5/QtQml/5.7.1/QtQml/private/qv4jsir_p.h:58:39: fatal error: QtCore/private/qnumeric_p.h: No such file or directory
testenv3_build/py3.5-qt5.7.1-64bit-release/pyside2/libpyside/signalmanager.cpp:65:37: fatal error: private/qv4engine_p.h: No such file or directory
The code should be fixed to do proper detection.