iOS Simulator 7.1 crash running on Yosemite with weak linked new frameworks (Symbol not found: _objc_isAuto)
This issue can occur if you are building your project against the iOS 8.x SDKs and are weak linking a framework that is new to iOS 8.x and also present in OS X 10.10 and you run in the iOS 7.1 simulator.
The Xcode 6.1 Release Notes mention this issue:
If an app is weak linked against frameworks new in iOS 8 SDK and OS X 10.10 SDK, it may fail to run if the run destination is an iOS Simulator for older iOS runtimes and the host system is running OS X Yosemite. (17807439)
The issue is resolved in the updates iOS 7.1 simulator runtime that can be downloaded from Xcode 6.2 beta 4 an later (and I think possibly beta3 worked as well).