Since updating to Xcode 9.3 our project crashes on launch for optimized builds. It happens both on device and simulator, but not necessarily in the same locations. It worked fine with the previous Xcode. It happens for all builds with -O or -Osize optimizations, independent of WMO de/activated, and independent of Swift 3 / Swift 4 mode.
Changing arbitrary things in the method makes it run without crash. Even putting it into a different file might make it not crash anymore.
Example repro code:
Example project for convenience: https://github.com/fabb/swift_4_1_crash