Writing notes as i test things. It seems that I can delete the contents of the file that it throws this issue for compiling and it still gives a compilation error for the file. Also, if i remove the reference from Xcode and re-add it, it still results in the same compile error for that file.

Deleting the file and creating a new file with the same name still results in a compilation error. Moving the file's location did not affect whether or not the compilation error occurred for that file.

I seem to consistently get the same number of files have compilation errors if i delete derived data before building. If I don't delete derived data, that number can change, and sometimes some of those files will compile successfully.

None of this makes any sense to me, hopefully it will to someone else 😛

That part's just how incremental builds work in Xcode 10. Files are compiled in batches, and in some cases information is emitted to all files in a batch.

Some possibly related Radars, for Apple people picking this up: rdar://problem/40949298&41430307&43406595

Ah, if you download a development snapshot (for master, not 4.2) there should be a bit more info in the build log. https://swift.org/download/#snapshots

Using that I now get a couple errors. Now it fails with: "Segmentation fault: 11"

The stack trace looks like:

2. While walking into decl 'FailingFileContainerViewController' at /Users/scottrobbins/Projects/ios/path/to/the/file/FailingFileContainerViewController.swift:1:1

0 swift 0x0000000105ee6288 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 40

1 swift 0x0000000105ee5515 llvm::sys::RunSignalHandlers() + 85

2 swift 0x0000000105ee6892 SignalHandler(int) + 258

3 libsystem_platform.dylib 0x00007fff76a38f5a _sigtramp + 26

4 libsystem_platform.dylib 0x00007fb77f5f7000 _sigtramp + 146530496

5 swift 0x00000001030d3b23 (anonymous namespace)::IndexSwiftASTWalker::startEntity(swift::Decl*, swift::index::IndexSymbol&, bool) + 371

6 swift 0x00000001030d1d3f (anonymous namespace)::IndexSwiftASTWalker::walkToDeclPre(swift::Decl*, swift::CharSourceRange) + 1119

7 swift 0x000000010308cf49 (anonymous namespace)::SemaAnnotator::walkToDeclPre(swift::Decl*) + 1209

8 swift 0x000000010365937b (anonymous namespace)::Traversal::doIt(swift::Decl*) + 171

9 swift 0x00000001036592bb swift::Decl::walk(swift::ASTWalker&) + 27

10 swift 0x00000001037124ca swift::SourceFile::walk(swift::ASTWalker&) + 170

11 swift 0x000000010308b249 swift::SourceEntityWalker::walk(swift::SourceFile&) + 201

12 swift 0x00000001030d1638 (anonymous namespace)::IndexSwiftASTWalker::visitModule(swift::ModuleDecl&, llvm::StringRef) + 248

13 swift 0x00000001030d1494 swift::index::indexSourceFile(swift::SourceFile*, llvm::StringRef, swift::index::IndexDataConsumer&) + 500

14 swift 0x00000001030da2a9 recordSourceFileUnit(swift::SourceFile*, llvm::StringRef, llvm::StringRef, bool, bool, llvm::StringRef, llvm::ArrayRef<clang::FileEntry const*>, clang::CompilerInstance const&, swift::DiagnosticEngine&) + 1657

15 swift 0x00000001030d9c1a swift::index::indexAndRecord(swift::SourceFile*, llvm::StringRef, llvm::StringRef, bool, bool, llvm::StringRef, swift::DependencyTracker const&) + 298

16 swift 0x000000010248fb3c emitIndexDataIfNeeded(swift::SourceFile*, swift::CompilerInvocation const&, swift::CompilerInstance&) + 284

17 swift 0x000000010248ad5c performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 15164

18 swift 0x00000001024861d4 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 2932

19 swift 0x000000010243b51d main + 685

20 libdyld.dylib 0x00007fff7672a015 start + 1

error: Segmentation fault: 11

(FailingFileContainerViewController is one of our classes)

The other error that shows up says: "Command CompileSwift failed with a nonzero exit code". No stack trace though, just the print out of what it ran.

Edit: If I comment out the code in that file, when i build they all complains about a different file, and so on.

All of these files so far seem to inherit from UIKit objects. (custom uiviewcontrollers, uitableviewcells, uiviews, etc)

I get this same segmentation fault just by inheriting from a UIKit object in the attatched test project, however that error doesn't occur with the Xcode 10 toolchain, just the snapshot of master, so it may be unrelated.

Yeah, the indexing thing sounds like a different problem. Maybe try turning off "Enable Index-While-Building Functionality" in your build settings for now?

I turned off "Enable Index-While-Building Functionality" as you suggested, and using the "Swift Development Snapshot 2018-09-10 (a)" the project successfully compiles.

This still fails on Xcode 10.1 beta 1

@belkadan , do you have any timeline as to when the fix on master could make it into an Xcode? And is there any further investigation you need?

Well, we don't know what fixed it on master, so if it's going to get into a release sooner than Swift 5 we'd have to track that down. Even then, it might not be a safe change to cherry-pick, but maybe we'd have a workaround, at least.

If you have free time, bisecting through master development snapshots to figure out when it was fixed would be helpful. Otherwise one of us will get to it at some point…unless we're close to the Swift 5 release by then.

(Note: I'm using "Swift 5" to mean "the next release that branches from master". As last spring showed, that might not actually be called "Swift 5".)

This seems to be fixed in Swift 5, testing with Xcode 10.2 beta 1