@_functionBuilder
public struct StringBuilder {
    public static func buildBlock(_ children: String...) -> String {
        return children.joined()
    }

    public static func buildIf(_ child: String?) -> String {
        return child ?? ""
    }
}

@StringBuilder
public func combinedString(spaced: Bool) -> String {
    "test"
    if spaced {
        " "
    }
    "other"
}

1.  While emitting SIL for 'combinedString(spaced:)' (at /Users/dan/Desktop/StringPattern/Sources/StringPattern/StringBuilder.swift:22:8)
0  swift                    0x00000001046e78a3 PrintStackTraceSignalHandler(void*) + 51
1  swift                    0x00000001046e7076 SignalHandler(int) + 358
2  libsystem_platform.dylib 0x00007fff68724b1d _sigtramp + 29
3  libsystem_platform.dylib 0x00007ffeef93c0c8 _sigtramp + 2267117000
4  swift                    0x0000000100c92187 swift::SILProfiler::create(swift::SILModule&, swift::ForDefinition_t, swift::ASTNode) + 3079
5  swift                    0x00000001007403f3 setUpForProfiling(swift::SILDeclRef, swift::SILFunction*, swift::ForDefinition_t) + 115
6  swift                    0x000000010073f503 swift::Lowering::SILGenModule::getFunction(swift::SILDeclRef, swift::ForDefinition_t) + 483
7  swift                    0x0000000100740b16 swift::Lowering::SILGenModule::emitFunction(swift::FuncDecl*) + 790
8  swift                    0x000000010074b456 swift::Lowering::SILGenModule::emitSourceFile(swift::SourceFile*) + 1238
9  swift                    0x000000010074cedc swift::SILModule::constructSIL(swift::ModuleDecl*, swift::SILOptions&, swift::FileUnit*) + 1244
10 swift                    0x0000000100359d75 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 28757
11 swift                    0x000000010034f424 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 6868
12 swift                    0x00000001002de6b3 main + 1219
13 libdyld.dylib            0x00007fff685247a9 start + 1
14 libdyld.dylib            0x0000000000000052 start + 2544744618
error: Segmentation fault: 11