Note to self: I’d a solution where Project A referenced Project B. This had been fine and stable for some time, and then suddenly intellisence started warning that the namespace I needed from B was missing, and that I was missing a project reference. Except I wasn’t. And the solution would build without fault.

I found suggestions of deleting /obj and /.vs folders – which I did – but the fix came from turning off Lightweight Solution Loading. See : https://developercommunity.visualstudio.com/content/problem/98484/project-references-do-not-work.html

Clear Visual Studio bug.