Visual Studio 2013 reporting erroneous "does not contain a definition for" error for recent extension methods

For Visual Studio 2015, my solution didn't have an .sdf file, and restarting didn't help. However, the following steps fixed my problem:

1. Close Visual Studio (important)
2. Go to [solution folder]\.vs\[solution name]\v14
3. Delete the .suo file
4. Start Visual Studio

It turns out that closing all open files, then shutting down Visual Studio, and restarting fixed the issue. pff!

I had a similar problem that I resolved by deleting *.sdf file in the solution directory. VS2013 will re-create it.

UPDATE This is what I know now: https://stackoverflow.com/a/38708050/90475