Xcode 6 linker error - Undefined symbols for architecture armv7

Question

After upgrading to Xcode 6 beta 7 (and now still with Xcode 6 GM) I am unable to link my Swift app. I receive errors such as:

Undefined symbols for archit

Answer 1

'Upgraded' to xcode 6 and swift app wouldn't build for simulator yet would build fine on an iPad - Linker error, undefined symbols for i386 for the simulator.

Clearing the Library/Developer/Xcode/DerivedData folder worked for me.