import Foundation
struct User: Decodable {}
protocol Transport {} 
struct X: Transport {}
// This line is key. Some types cause a crash; some do not.
// URLSession, NSObject, NSNumber – crash
// NSArray, NSProxy, URL, NSMeasurement – no crash
// Swift NSObject subclass – no crash
extension URLSession: Transport {}
// fetch must be generic over a restricted type (Decodable, Hashable both crash)
func fetch<Model: Decodable>(_ model: Model.Type, with transport: Transport) {}
// Note that X is passed here; a standard Swift struct unrelated to the objc type
fetch(User.self, with: X())

generic witness table at 0x10c1304b4 contains out-of-bounds requirement descriptor 0x10c1304ca
 Stack dump:
 0. Program arguments: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift -frontend -interpret x.swift -enable-objc-interop -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk -color-diagnostics -module-name x
 0 swift 0x0000000106ba6ee3 PrintStackTraceSignalHandler(void*) + 51
 1 swift 0x0000000106ba66bc SignalHandler(int) + 348
 2 libsystem_platform.dylib 0x00007fff70f80b5d _sigtramp + 29
 3 libsystem_platform.dylib 0x0000000000000004 _sigtramp + 2399663300
 4 libsystem_c.dylib 0x00007fff70e406a6 abort + 127
 5 libswiftCore.dylib 0x00007fff707b8ed5 swift::fatalError(unsigned int, char const*, ...) + 149
 6 libswiftCore.dylib 0x00007fff707c812a swift_getWitnessTable + 1994
 7 libswiftCore.dylib 0x00000001081672cc swift_getWitnessTable + 2543450476
 8 libswiftCore.dylib 0x00000001081670cd swift_getWitnessTable + 2543449965
 9 swift 0x000000010343e19d llvm::MCJIT::runFunction(llvm::Function*, llvm::ArrayRef<llvm::GenericValue>) + 365
 10 swift 0x0000000103444572 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*) + 1090
 11 swift 0x0000000102a0d5d1 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 58913
 12 swift 0x00000001029fb6de swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 6862
 13 swift 0x00000001029997be main + 1246
 14 libdyld.dylib 0x00007fff70d9b3d5 start + 1
 Abort trap: 6