Since Qt5 the recommendation for Linux distributions has been to use the new qtchooser tool from Thiago. It fills /usr/bin/* with Qt development binaries, and provides the wanted Qt version according to its configuration. Debian and Ubuntu have migrated to that solution now.

Currently however Qt Creator uses a detection order that makes Qt 4 the toolkit to use in Qt Creator on first run even if Qt 5 is the default in qtchooser configuration. This is because the compatibility 'qmake-qt4' still exists, and is checked first. Instead of that way, /usr/bin/qmake should be made the preferred qmake to check, falling back to suffixes if it's not found.

1. If only Qt 5 is installed and is the default in qtchooser configuration, it's correctly detected by Qt Creator.
2. If both Qt 4 and Qt 5 are installed, Qt Creator should prefer the one that has been selected as the default in qtchooser configuration. This isn't working now.
3. Only if /usr/bin/qmake does not exist at all, it should fallback to search qmake-qt4, qmake-qt5 etc.

(I'll submit a change request that fixes the 2. for me while keeping 1. and 3. working)