Bug case 1249294
I’ve just updated VS to 16.6 with a hope of fixing ECS Intellisense malfunctioning issue, but it turned out to break Unity project like so:
This happens with both Unity 2020.1.0b5 and 0b9 versions of Unity. Anyone stumbled upon this issue?
I haven’t had the “needs migration” message, but with newer versions of visual studio (and then vs code) it wouldn’t be able to see things like the Unity namespace until I regenerated the project files
That was one of the many things I’ve tried but with no luck
Thanks for the bug report, QA will have a look.
After couple of emails with support team, and their inability to reproduce this problem, I decided to completely uninstall Unity and VS 2019, delete all I could find related to Unity and Unity Hub on my machine, then open registry editor, and again remove everything related to Unity and Unity Hub…then install Unity b12 via Unity Hub and VS 16.6.2 via vs installer…but the problem was still there. Then I manually, once again, removed all .csproj and .sln files and forced recreation from External Tools. I was finally able to open a solution in VS.
Note that both, VS and Unity were updated twice since I opened this bug, so it might be that it has been solved.
Interesting fact, whatever I tryed, I could not make Unity Hub log me out on reinstall. And I tried all sorts of things and cleaned all the traces of it I could find. Whatever dark magic you are using, it’s not fair.
The “needs migration” and broken Intellisense in Visual Studio happened to me today.
Tried to regenerate projects files as suggested in the forums, but that did not help
(Unity > Edit > Preferences… > External Tools > Regenerate project files)
I think I caused it by uninstalling and reinstalling the Visual Studio tools for Unity while trying to fix a different issue.
(Visual Studio 2019 > Tools > Get Tool & Features > Individual Components > Visual Studio 2019 Tools for Unity)
After reinstalling the extension it turned out to still be disabled 
so I had to enable it:
Visual Studio 2019 > Extensions > Manage Extensions > Installed > Visual Studio 2019 Tools for Unity > Enable > Restart VS
Crap…that was probably my problem also, but I did quite a workaround.