$ cat test.swift
enum Whatever { case Thing }
func test(x: Whatever) {
  switch x {
    case .Thing:
      break
    @unknown case _:
      break
    @unknown case _:
      break
  }
}

$ ../build/Ninja-ReleaseAssert/swift-macosx-x86_64/bin/swift test.swift
test.swift:6:14: error: '@unknown' can only be applied to the last case in a switch
    @unknown case _:
             ^
Assertion failed: (unknownCase == nullptr), function checkExhaustiveness, file /Users/rintaro/Repositories/swift-oss/swift/lib/Sema/TypeCheckSwitchStmt.cpp, line 1219.
0  swift                    0x0000000113831898 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 40
1  swift                    0x0000000113831f26 SignalHandler(int) + 422
2  libsystem_platform.dylib 0x00007fff602e8f5a _sigtramp + 26
3  swift                    0x0000000114b970d0 cmark_strbuf__initbuf + 113003
4  libsystem_c.dylib        0x00007fff600861ae abort + 127
5  libsystem_c.dylib        0x00007fff6004e1ac basename_r + 0
6  swift                    0x0000000110d876f3 swift::TypeChecker::checkSwitchExhaustiveness(swift::SwitchStmt const*, swift::DeclContext const*, bool) + 6595
7  swift                    0x0000000110d8437f (anonymous namespace)::StmtChecker::visitSwitchStmt(swift::SwitchStmt*) + 3215
8  swift                    0x0000000110d81d6c swift::ASTVisitor<(anonymous namespace)::StmtChecker, void, swift::Stmt*, void, void, void, void>::visit(swift::Stmt*) + 204
9  swift                    0x0000000110d801b8 swift::TypeChecker::typeCheckFunctionBodyUntil(swift::FuncDecl*, swift::SourceLoc) + 328
10 swift                    0x0000000110d80e71 swift::TypeChecker::typeCheckAbstractFunctionBody(swift::AbstractFunctionDecl*) + 449
11 swift                    0x0000000110da6065 typeCheckFunctionsAndExternalDecls(swift::SourceFile&, swift::TypeChecker&) + 261
12 swift                    0x0000000110da6ff8 swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int, unsigned int, unsigned int) + 1816
13 swift                    0x00000001109dfd90 swift::CompilerInstance::parseAndTypeCheckMainFile(swift::PersistentParserState&, swift::DelayedParsingCallbacks*, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>) + 416
14 swift                    0x00000001109dea52 swift::CompilerInstance::parseAndCheckTypes(swift::CompilerInstance::ImplicitImports const&) + 626
15 swift                    0x00000001109de3b2 swift::CompilerInstance::performSema() + 466
16 swift                    0x000000010fe1bb90 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 1904
17 swift                    0x000000010fe1a40e swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 3310
18 swift                    0x000000010fdd8863 main + 2051
19 libdyld.dylib            0x00007fff5ffda015 start + 1
20 libdyld.dylib            0x000000000000000a start + 2684510198
Stack dump:
0.  Program arguments: /Users/rintaro/Repositories/swift-oss/build/Ninja-ReleaseAssert/swift-macosx-x86_64/bin/swift -frontend -interpret test.swift -enable-objc-interop -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk -color-diagnostics -module-name test 
1.  While type-checking 'test(x:)' at test.swift:2:1
Abort trap: 6