Steps to reproduce:

1. Open attached project
2. Open scene "scene"
3. Open script "Scripts/NewBehaviourScript" and add breakpoint at debugger statement
4. Attach VS debugger to Editor and run the scene
5. Once the breakpoint is hit in VS Watch window add expression "1.0f - 1"
6. Notice that this expression isn't supported

Doesn't reproduce with MonoDevelop or standalone VS application

Reproduced with VSTU 2.3.0.0 and VS Community Version 14.0.25425.01 Update 3
5.3.7f1, 5.5.0b11, 5.6.0a2