import Foundation

class Test: NSObject {
    let value: Int? = nil
}

var obj = Test()
let _ = obj.observe(\.value) { _, _ in }

fatal error: Could not extract a String from KeyPath Swift.WritableKeyPath<test.Test, Swift.Optional<Swift.Int>>: file /Library/Caches/com.apple.xbs/Sources/swiftlang/swiftlang-900.0.43/src/swift/stdlib/public/SDK/Foundation/NSObject.swift, line 85
0  swift                    0x00000001077fe3aa PrintStackTraceSignalHandler(void*) + 42
1  swift                    0x00000001077fd7e6 SignalHandler(int) + 662
2  libsystem_platform.dylib 0x00007fffd5536b3a _sigtramp + 26
3  libsystem_platform.dylib 000000000000000000 _sigtramp + 715953376
4  libswiftFoundation.dylib 0x000000010d643972 _T010Foundation26_KVOKeyPathBridgeMachinery33_6DA0945A07226B3278459E9368612FF4LLC010_bridgeKeyC0SSs03AnylC0CFZTf4gd_n + 594
5  libswiftFoundation.dylib 0x000000010d5aad61 _T010Foundation27_KeyValueCodingAndObservingPAAE7observeAA05NSKeyC11ObservationCs0B4PathCyxqd__G_So0hcF7OptionsV7optionsyx_AA0hC14ObservedChangeVyqd__Gtc13changeHandlertlF + 321
6  libswiftFoundation.dylib 0x000000010a24d103 _T010Foundation27_KeyValueCodingAndObservingPAAE7observeAA05NSKeyC11ObservationCs0B4PathCyxqd__G_So0hcF7OptionsV7optionsyx_AA0hC14ObservedChangeVyqd__Gtc13changeHandlertlF + 4241106147
7  swift                    0x0000000104dbf74f llvm::MCJIT::runFunction(llvm::Function*, llvm::ArrayRef<llvm::GenericValue>) + 655
8  swift                    0x0000000104dc59b3 llvm::ExecutionEngine::runFunctionAsMain(llvm::Function*, std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&, char const* const*) + 707
9  swift                    0x00000001041d2374 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 22532
10 swift                    0x00000001041cb304 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 7332
11 swift                    0x0000000104181728 main + 12248
12 libdyld.dylib            0x00007fffd5327235 start + 1
13 libdyld.dylib            0x000000000000000c start + 718114264
Stack dump:
0.  Program arguments: /Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift -frontend -interpret ../test.swift -enable-objc-interop -sdk /Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk -swift-version 4 -color-diagnostics -module-name test
[1]    53047 illegal hardware instruction  DEVELOPER_DIR=/Applications/Xcode-beta.app/Contents/Developer xcrun swift  4