If there is a space in the relative path between the shadow build directory and the source directory then the user-created sources will not be built.

Steps to reproduce:
Create the following follwing path of folders:
"C:\test\test2\folder with spaces"
Create a project inside qt creator and place it in "folder with spaces" above.
The following should be the default shadow build path:
C:\test\test2\folder with spaces\build-qt_folder_bug-Desktop_Qt_5_6_1_MSVC2015_32bit-Debug

Build the project, this will succeed.

In project settings change the shadow build directory to
C:\test\test2\build-qt_folder_bug-Desktop_Qt_5_6_1_MSVC2015_32bit-Debug

Build the project, it will fail and complain that main.obj isn't found. No non-moc sources will be built.