Qmake builds for MSVC.

The CONFIG += warn_on and warn_off work just fine. That's why I'm using it. The issue is that qmake parses the command line arguments (MSVC allows -W4 and /W4, they mean the same thing) and coverts them to settings. However, it doesn't seem to let you override the warn_[on|off] options with something explicit on the command line, like -W4, which, for example, cmake does, cause it parses all of the arguments you give it and it selects the right project options for MSVC when doing that.