Steps to reproduce:
1. Create a new empty project
2. Go to "Player Settings"
3. Set "Scripting Runtime Version" to "Experimental (.NET 4.6 Equivalent)
4. Unity will request to restart the Editor > accept.
5. Try to reopen project

Result: Unity crashes. Unable to open project.

Reproduced with: 2017.2.0b2, 2017.2.0b1.
Not reproduced with: 2017.2.0a4, 2017.1.0f2 and earlier.

Note:
Deleting Temp and Library folders does not help.
Reproduced on macOS.
Works as expected on Windows.