Add cross-compilation regression test

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More cleanup

Allow skipping canSwiftPMCompileForIOS on CI

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Author: kabiroberai

Sources/SKTestSupport/SkipUnless.swift

@@ -477,6 +477,34 @@ package actor SkipUnless {
     }
   }
 
+  package static func canSwiftPMCompileForIOS(
+    file: StaticString = #filePath,
+    line: UInt = #line
+  ) async throws {
+    return try await shared.skipUnlessSupported(allowSkippingInCI: true, file: file, line: line) {
+      #if os(macOS)
+      let project = try await SwiftPMTestProject(files: [
+        "MyFile.swift": """
+        public func foo() {}
+        """
+      ])
+      do {
+        try await SwiftPMTestProject.build(
+          at: project.scratchDirectory,
+          extraArguments: [
+            "--swift-sdk", "arm64-apple-ios",
+          ]
+        )
+        return .featureSupported
+      } catch {
+        return .featureUnsupported(skipMessage: "Cannot build for iOS: \(error)")
+      }
+      #else
+      return .featureUnsupported(skipMessage: "Cannot build for iOS outside macOS by default")
+      #endif
+    }
+  }
+
   package static func canCompileForWasm(
     file: StaticString = #filePath,
     line: UInt = #line

Tests/SourceKitLSPTests/BackgroundIndexingTests.swift

@@ -1018,6 +1018,53 @@ final class BackgroundIndexingTests: XCTestCase {
     )
   }
 
+  func testUseSwiftSDKFlagsDuringPreparation() async throws {
+    try await SkipUnless.canSwiftPMCompileForIOS()
+
+    var options = SourceKitLSPOptions.testDefault()
+    options.swiftPMOrDefault.swiftSDK = "arm64-apple-ios"
+    let project = try await SwiftPMTestProject(
+      files: [
+        "Lib/Lib.swift": """
+        #if os(iOS)
+        public func foo() -> Int { 1 }
+        #endif
+        """,
+        "Client/Client.swift": """
+        import Lib
+
+        func test() -> String {
+          return foo()
+        }
+        """,
+      ],
+      manifest: """
+        let package = Package(
+          name: "MyLibrary",
+          targets: [
+            .target(name: "Lib"),
+            .target(name: "Client", dependencies: ["Lib"]),
+          ]
+        )
+        """,
+      options: options,
+      enableBackgroundIndexing: true
+    )
+
+    // Check that we get an error about the return type of `foo` (`Int`) not being convertible to the return type of
+    // `test` (`String`), which indicates that `Lib` had `foo` and was thus compiled for iOS
+    let (uri, _) = try project.openDocument("Client.swift")
+    let diagnostics = try await project.testClient.send(
+      DocumentDiagnosticsRequest(textDocument: TextDocumentIdentifier(uri))
+    )
+    XCTAssert(
+      (diagnostics.fullReport?.items ?? []).contains(where: {
+        $0.message == "Cannot convert return expression of type 'Int' to return type 'String'"
+      }),
+      "Did not get expected diagnostic: \(diagnostics)"
+    )
+  }
+
   func testLibraryUsedByExecutableTargetAndPackagePlugin() async throws {
     try await SkipUnless.swiftPMStoresModulesForTargetAndHostInSeparateFolders()
     let project = try await SwiftPMTestProject(