Ignore the --testing-library argument we are going to start passing from SwiftPM.

Sources/XCTest/Private/ArgumentParser.swift

@@ -50,8 +50,37 @@ internal struct ArgumentParser {
 
     private let arguments: [String]
 
+    /// Filter out arguments from an arguments array that XCTest doesn't care about.
+    ///
+    /// - Parameters:
+    ///     - arguments: The arguments array to filter.
+    ///
+    /// - Returns: A copy of `arguments` with ignored arguments removed.
+    private static func removeIgnoredArguments(from arguments: [String]) -> [String] {
+        var result = [String]()
+        result.reserveCapacity(arguments.count)
+
+        var iterator = arguments.makeIterator()
+        while let argument = iterator.next() {
+            // Filter out any arguments of the form "--testing-library=xctest".
+            if argument.starts(with: "--testing-library=") {
+                continue
+            }
+
+            // Next, filter out any split arguments ("--testing-library xctest")
+            if argument == "--testing-library" {
+                _ = iterator.next() // skip subsequent value
+                continue
+            }
+
+            result.append(argument)
+        }
+
+        return result
+    }
+
     init(arguments: [String]) {
-        self.arguments = arguments
+        self.arguments = Self.removeIgnoredArguments(from: arguments)
     }
 
     var executionMode: ExecutionMode {

Tests/Functional/ArgumentParserWithIgnoredArgs/main.swift

@@ -0,0 +1,22 @@
+// RUN: %{swiftc} %s -o %T/ArgumentParserWithIgnoredArgs
+// RUN: %T/ArgumentParserWithIgnoredArgs
+
+#if os(macOS)
+    import SwiftXCTest
+#else
+    import XCTest
+#endif
+
+class ArgumentParsingWithIgnoredArgsTestCase: XCTestCase {
+    static var allTests = {
+        return [
+          ("testFail", testFail),
+          ("testSuccess", testSuccess),
+        ]
+    }()
+    func testFail() { XCTFail("failure") }
+    func testSuccess() { }
+}
+
+let arguments = ["main", "--testing-library=xctest", "--testing-library", "xctest", "\(String(reflecting: ArgumentParsingWithIgnoredArgsTestCase.self))/testSuccess", "--testing-library",]
+XCTMain([testCase(ArgumentParsingWithIgnoredArgsTestCase.allTests)], arguments: arguments)