diff --git a/Package.resolved b/Package.resolved
index ac259d52..2fd123a0 100644
--- a/Package.resolved
+++ b/Package.resolved
@@ -15,7 +15,7 @@
         "repositoryURL": "https://github.com/apple/swift-llbuild.git",
         "state": {
           "branch": "master",
-          "revision": "40c302a0b2ac12e99d1355986e4f16590d1fb03e",
+          "revision": "0615bdec228d6ed5be4ffa9ae00de0c5e0cb937c",
           "version": null
         }
       },
@@ -24,7 +24,7 @@
         "repositoryURL": "https://github.com/apple/swift-package-manager.git",
         "state": {
           "branch": "master",
-          "revision": "70829165806e77c2d0b72aa172cc6e1af7fa94e4",
+          "revision": "65900fb1355c6f3b8ee57dd43ddb57d5cc66da30",
           "version": null
         }
       }
diff --git a/Package.swift b/Package.swift
index d5a0be77..7835f73e 100644
--- a/Package.swift
+++ b/Package.swift
@@ -38,6 +38,9 @@ let package = Package(
       .target(
         name: "SKSwiftPMWorkspace",
         dependencies: ["SwiftPM", "SKCore"]),
+      .testTarget(
+        name: "SKSwiftPMWorkspaceTests",
+        dependencies: ["SKSwiftPMWorkspace", "SKTestSupport"]),
 
       // Csourcekitd: C modules wrapper for sourcekitd.
       .target(
diff --git a/Sources/SKSwiftPMWorkspace/SwiftPMWorkspace.swift b/Sources/SKSwiftPMWorkspace/SwiftPMWorkspace.swift
index 844adca6..d9281a66 100644
--- a/Sources/SKSwiftPMWorkspace/SwiftPMWorkspace.swift
+++ b/Sources/SKSwiftPMWorkspace/SwiftPMWorkspace.swift
@@ -74,7 +74,6 @@ public final class SwiftPMWorkspace {
     // FIXME: duplicating code from UserToolchain setup in swiftpm.
     var sdkpath: AbsolutePath? = nil
     var platformPath: AbsolutePath? = nil
-    let target: Triple = Triple.hostTriple
     if case .darwin? = Platform.currentPlatform {
       if let path = try? Process.checkNonZeroExit(args: "/usr/bin/xcrun", "--show-sdk-path", "--sdk", "macosx") {
         sdkpath = try? AbsolutePath(validating: path.spm_chomp())
@@ -84,8 +83,8 @@ public final class SwiftPMWorkspace {
       }
     }
 
-    var extraSwiftFlags = ["-target", target.tripleString]
-    var extraClangFlags = ["-arch", target.arch.rawValue]
+    var extraSwiftFlags: [String] = []
+    var extraClangFlags: [String] = []
     if let sdkpath = sdkpath {
       extraSwiftFlags += [
         "-sdk", sdkpath.asString
@@ -344,16 +343,23 @@ extension ToolchainRegistry {
 
   /// A toolchain appropriate for using to load swiftpm manifests.
   fileprivate var swiftpmHost: SwiftPMToolchain? {
-    guard let base = self.default, base.swiftc != nil else {
+    var swiftc: AbsolutePath? = self.default?.swiftc
+    var clang: AbsolutePath? = self.default?.clang
+    if swiftc == nil {
+      swiftc = toolchains.first(where: { $0.swiftc != nil })?.swiftc
+    }
+    if clang == nil {
+      clang = toolchains.first(where: { $0.clang != nil })?.clang
+    }
+
+    if swiftc == nil || clang == nil {
       return nil
     }
 
-    guard let clang = base.clang ?? toolchains.first(where: { $0.clang != nil })?.clang else { return nil }
-
     return SwiftPMToolchain(
-      swiftCompiler: base.swiftc!,
-      clangCompiler: clang,
-      libDir: base.swiftc!.parentDirectory.parentDirectory.appending(components: "lib", "swift", "pm"),
+      swiftCompiler: swiftc!,
+      clangCompiler: clang!,
+      libDir: swiftc!.parentDirectory.parentDirectory.appending(components: "lib", "swift", "pm"),
       sdkRoot: nil,
       extraCCFlags: [],
       extraSwiftCFlags: [],
diff --git a/Sources/SKTestSupport/FileSystem.swift b/Sources/SKTestSupport/FileSystem.swift
new file mode 100644
index 00000000..91a49ff2
--- /dev/null
+++ b/Sources/SKTestSupport/FileSystem.swift
@@ -0,0 +1,29 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2018 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import Basic
+
+extension FileSystem {
+
+  /// Creates files from a dictionary of path to contents.
+  ///
+  /// - parameters:
+  ///   - root: The root directory that the paths are relative to.
+  ///   - files: Dictionary from path (relative to root) to contents.
+  public func createFiles(root: AbsolutePath = .root, files: [String: ByteString]) throws {
+    for (path, contents) in files {
+      let path = AbsolutePath(path, relativeTo: root)
+      try createDirectory(path.parentDirectory, recursive: true)
+      try writeFileContents(path, bytes: contents)
+    }
+  }
+}
diff --git a/Tests/LinuxMain.swift b/Tests/LinuxMain.swift
index eb8321bf..fdf0f74f 100644
--- a/Tests/LinuxMain.swift
+++ b/Tests/LinuxMain.swift
@@ -4,6 +4,7 @@ import LanguageServerProtocolJSONRPCTests
 import LanguageServerProtocolTests
 import SKCoreTests
 import SKSupportTests
+import SKSwiftPMWorkspaceTests
 import SourceKitTests
 
 var tests = [XCTestCaseEntry]()
@@ -11,6 +12,7 @@ tests += LanguageServerProtocolJSONRPCTests.__allTests()
 tests += LanguageServerProtocolTests.__allTests()
 tests += SKCoreTests.__allTests()
 tests += SKSupportTests.__allTests()
+tests += SKSwiftPMWorkspaceTests.__allTests()
 tests += SourceKitTests.__allTests()
 
 XCTMain(tests)
diff --git a/Tests/SKSwiftPMWorkspaceTests/SwiftPMWorkspaceTests.swift b/Tests/SKSwiftPMWorkspaceTests/SwiftPMWorkspaceTests.swift
new file mode 100644
index 00000000..7665bc8b
--- /dev/null
+++ b/Tests/SKSwiftPMWorkspaceTests/SwiftPMWorkspaceTests.swift
@@ -0,0 +1,222 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2018 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import SKSwiftPMWorkspace
+import SKCore
+import PackageModel
+import Basic
+import SKTestSupport
+import XCTest
+
+final class SwiftPMWorkspaceTests: XCTestCase {
+  func testBasicSwiftArgs() {
+    // FIXME: should be possible to use InMemoryFileSystem.
+    let fs = localFileSystem
+    let tempDir = try! TemporaryDirectory(removeTreeOnDeinit: true)
+    try! fs.createFiles(root: tempDir.path, files: [
+      "pkg/Sources/lib/a.swift": "",
+      "pkg/Package.swift": """
+          // swift-tools-version:4.2
+          import PackageDescription
+          let package = Package(name: "a", products: [], dependencies: [],
+            targets: [.target(name: "lib", dependencies: [])])
+          """
+    ])
+    let packageRoot = tempDir.path.appending(component: "pkg")
+    let tr = ToolchainRegistry.shared
+    let ws = try! SwiftPMWorkspace(
+      workspacePath: packageRoot,
+      toolchainRegistry: tr,
+      fileSystem: fs)!
+
+    let aswift = packageRoot.appending(components: "Sources", "lib", "a.swift")
+
+    XCTAssertEqual(ws.buildPath, packageRoot.appending(components: ".build", "debug"))
+    XCTAssertNotNil(ws.indexStorePath)
+    let arguments = ws.settings(for: aswift.asURL, language: .swift)!.compilerArguments
+
+    check(
+      "-module-name", "lib", "-incremental", "-emit-dependencies",
+      "-emit-module", "-emit-module-path", arguments: arguments)
+    check("-parse-as-library", "-c", arguments: arguments)
+
+    check("-target", arguments: arguments) // Only one!
+#if os(macOS)
+    check("-target", "x86_64-apple-macosx10.10", arguments: arguments)
+    check("-sdk", arguments: arguments)
+    check("-F", arguments: arguments)
+#else
+    check("-target", "x86_64-unknown-linux", arguments: arguments)
+#endif
+
+    check("-I", packageRoot.appending(components: ".build", "debug").asString, arguments: arguments)
+
+    check(aswift.asString, arguments: arguments)
+  }
+
+  func testMultiFileSwift() {
+    // FIXME: should be possible to use InMemoryFileSystem.
+    let fs = localFileSystem
+    let tempDir = try! TemporaryDirectory(removeTreeOnDeinit: true)
+    try! fs.createFiles(root: tempDir.path, files: [
+      "pkg/Sources/lib/a.swift": "",
+      "pkg/Sources/lib/b.swift": "",
+      "pkg/Package.swift": """
+          // swift-tools-version:4.2
+          import PackageDescription
+          let package = Package(name: "a", products: [], dependencies: [],
+            targets: [.target(name: "lib", dependencies: [])])
+          """
+    ])
+    let packageRoot = tempDir.path.appending(component: "pkg")
+    let tr = ToolchainRegistry.shared
+    let ws = try! SwiftPMWorkspace(
+      workspacePath: packageRoot,
+      toolchainRegistry: tr,
+      fileSystem: fs)!
+
+    let aswift = packageRoot.appending(components: "Sources", "lib", "a.swift")
+    let bswift = packageRoot.appending(components: "Sources", "lib", "b.swift")
+
+    let argumentsA = ws.settings(for: aswift.asURL, language: .swift)!.compilerArguments
+    check(aswift.asString, arguments: argumentsA)
+    check(bswift.asString, arguments: argumentsA)
+    let argumentsB = ws.settings(for: aswift.asURL, language: .swift)!.compilerArguments
+    check(aswift.asString, arguments: argumentsB)
+    check(bswift.asString, arguments: argumentsB)
+  }
+
+  func testMultiTargetSwift() {
+    // FIXME: should be possible to use InMemoryFileSystem.
+    let fs = localFileSystem
+    let tempDir = try! TemporaryDirectory(removeTreeOnDeinit: true)
+    try! fs.createFiles(root: tempDir.path, files: [
+      "pkg/Sources/libA/a.swift": "",
+      "pkg/Sources/libB/b.swift": "",
+      "pkg/Sources/libC/include/libC.h": "",
+      "pkg/Sources/libC/libC.c": "",
+      "pkg/Package.swift": """
+          // swift-tools-version:4.2
+          import PackageDescription
+          let package = Package(name: "a", products: [], dependencies: [],
+            targets: [
+              .target(name: "libA", dependencies: ["libB", "libC"]),
+              .target(name: "libB", dependencies: []),
+              .target(name: "libC", dependencies: []),
+            ])
+          """
+    ])
+    let packageRoot = tempDir.path.appending(component: "pkg")
+    let tr = ToolchainRegistry.shared
+    let ws = try! SwiftPMWorkspace(
+      workspacePath: packageRoot,
+      toolchainRegistry: tr,
+      fileSystem: fs)!
+
+    let aswift = packageRoot.appending(components: "Sources", "libA", "a.swift")
+    let bswift = packageRoot.appending(components: "Sources", "libB", "b.swift")
+    let arguments = ws.settings(for: aswift.asURL, language: .swift)!.compilerArguments
+    check(aswift.asString, arguments: arguments)
+    checkNot(bswift.asString, arguments: arguments)
+    check("-I", packageRoot.appending(components: "Sources", "libC", "include").asString, arguments: arguments)
+
+    let argumentsB = ws.settings(for: bswift.asURL, language: .swift)!.compilerArguments
+    check(bswift.asString, arguments: argumentsB)
+    checkNot(aswift.asString, arguments: argumentsB)
+    checkNot("-I", packageRoot.appending(components: "Sources", "libC", "include").asString, arguments: argumentsB)
+  }
+
+  func testBasicCXXArgs() {
+    // FIXME: should be possible to use InMemoryFileSystem.
+    let fs = localFileSystem
+    let tempDir = try! TemporaryDirectory(removeTreeOnDeinit: true)
+    try! fs.createFiles(root: tempDir.path, files: [
+      "pkg/Sources/lib/a.cpp": "",
+      "pkg/Sources/lib/b.cpp": "",
+      "pkg/Sources/lib/include/a.h": "",
+      "pkg/Package.swift": """
+          // swift-tools-version:4.2
+          import PackageDescription
+          let package = Package(name: "a", products: [], dependencies: [],
+            targets: [.target(name: "lib", dependencies: [])],
+            cxxLanguageStandard: .cxx14)
+          """
+    ])
+    let packageRoot = tempDir.path.appending(component: "pkg")
+    let tr = ToolchainRegistry.shared
+    let ws = try! SwiftPMWorkspace(
+      workspacePath: packageRoot,
+      toolchainRegistry: tr,
+      fileSystem: fs)!
+
+    let acxx = packageRoot.appending(components: "Sources", "lib", "a.cpp")
+    let bcxx = packageRoot.appending(components: "Sources", "lib", "b.cpp")
+    let build = packageRoot.appending(components: ".build", "debug")
+
+    XCTAssertEqual(ws.buildPath, build)
+    XCTAssertNotNil(ws.indexStorePath)
+    let arguments = ws.settings(for: acxx.asURL, language: .cpp)!.compilerArguments
+
+    check("-MD", "-MT", "dependencies",
+      "-MF", build.appending(components: "lib.build", "a.cpp.d").asString,
+      arguments: arguments)
+    check("-std=c++14", arguments: arguments)
+
+    checkNot("-arch", arguments: arguments)
+    check("-target", arguments: arguments) // Only one!
+#if os(macOS)
+    check("-target", "x86_64-apple-macosx10.10", arguments: arguments)
+    check("-isysroot", arguments: arguments)
+    check("-F", arguments: arguments)
+#else
+    check("-target", "x86_64-unknown-linux", arguments: arguments)
+#endif
+
+    check("-I", packageRoot.appending(components: "Sources", "lib", "include").asString, arguments: arguments)
+    checkNot("-I", build.asString, arguments: arguments)
+    check("-c", acxx.asString, arguments: arguments)
+    checkNot(bcxx.asString, arguments: arguments)
+    check("-o", build.appending(components: "lib.build", "a.cpp.o").asString, arguments: arguments)
+  }
+}
+
+private func checkNot(
+  _ pattern: String...,
+  arguments: [String],
+  file: StaticString = #file,
+  line: UInt = #line)
+{
+  if let index = arguments.firstIndex(of: pattern) {
+    XCTFail(
+      "not-pattern \(pattern) unexpectedly found at \(index) in arguments \(arguments)",
+      file: file, line: line)
+    return
+  }
+}
+
+private func check(
+  _ pattern: String...,
+  arguments: [String],
+  file: StaticString = #file,
+  line: UInt = #line)
+{
+  guard let index = arguments.firstIndex(of: pattern) else {
+    XCTFail("pattern \(pattern) not found in arguments \(arguments)", file: file, line: line)
+    return
+  }
+
+  if let index2 = arguments[(index+1)...].firstIndex(of: pattern) {
+    XCTFail(
+      "pattern \(pattern) found twice (\(index), \(index2)) in \(arguments)",
+      file: file, line: line)
+  }
+}
diff --git a/Tests/SKSwiftPMWorkspaceTests/XCTestManifests.swift b/Tests/SKSwiftPMWorkspaceTests/XCTestManifests.swift
new file mode 100644
index 00000000..0f803a53
--- /dev/null
+++ b/Tests/SKSwiftPMWorkspaceTests/XCTestManifests.swift
@@ -0,0 +1,21 @@
+#if !canImport(ObjectiveC)
+import XCTest
+
+extension SwiftPMWorkspaceTests {
+    // DO NOT MODIFY: This is autogenerated, use:
+    //   `swift test --generate-linuxmain`
+    // to regenerate.
+    static let __allTests__SwiftPMWorkspaceTests = [
+        ("testBasicCXXArgs", testBasicCXXArgs),
+        ("testBasicSwiftArgs", testBasicSwiftArgs),
+        ("testMultiFileSwift", testMultiFileSwift),
+        ("testMultiTargetSwift", testMultiTargetSwift),
+    ]
+}
+
+public func __allTests() -> [XCTestCaseEntry] {
+    return [
+        testCase(SwiftPMWorkspaceTests.__allTests__SwiftPMWorkspaceTests),
+    ]
+}
+#endif