1. Have Qt 5.9 from the online installer.
2. Configure a kit in Creator which uses this Qt version.
3. Open test-project "simple" from Creator's sources.
   It's in tests\manual\debugger\simple\simple.pro. Parsing the project will fail:

   Project ERROR: msvc-version.conf loaded but QMAKE_MSC_VER isn't set
   Error while parsing file C:\Users\rob\dev\src\creator-4.3-enterprise\qtcreator\tests\manual\debugger\simple\simple.pro. Giving up.
   

4. Switch to a different build configuration of the same kit.
   Now parsing the project will succeed.

Parsing the project should already succeed on first try. With Qt 5.8, this works correctly. Running qmake on the project from the command-line succeeds, too.

This seems to be caused by some change in Qt 5.9, but since the second parse passes and qmake behaves correctly, in also seems to point to some problem in Creator.