Run the Cxx interop check on every Linux version

.github/workflows/package_test.yml

@@ -49,7 +49,7 @@ on:
         default: true
       cxx_interop_swift_versions:
         type: string
-        description: "Swift version list (JSON array) for the Cxx interop check. Empty uses the newest release version in linux_swift_versions."
+        description: "Swift version list (JSON array) for the Cxx interop check. Empty uses linux_swift_versions."
         default: ""
 
       # ----- Version lists (JSON arrays) -----

.github/workflows/scripts/matrix/generate-matrix.swift

@@ -233,15 +233,10 @@ extension Configuration {
     return self.macOSVersions
   }
 
-  /// The Swift versions the Cxx interop check runs.
-  ///
-  /// The check is supplementary rather than a full compatibility check, so it runs on the
-  /// newest release in the Linux list unless a caller names more.
+  /// The Swift versions the Cxx interop check runs: those the Linux tests run, unless a caller
+  /// names others.
   var cxxInteropSwiftVersions: [String] {
-    if self.cxxInteropVersions.isEmpty {
-      return [SwiftVersion.newestRelease(in: self.linuxVersions, releases: self.releases)]
-    }
-    return self.cxxInteropVersions
+    self.cxxInteropVersions.isEmpty ? self.linuxVersions : self.cxxInteropVersions
   }
 
   /// The timeout an entry on this platform carries, or nil to leave the executor's own.
@@ -1634,14 +1629,6 @@ struct SwiftVersion: Comparable {
     }
     return version
   }
-
-  /// The newest release a version list holds, falling back to its last entry when the list is
-  /// all nightlies.
-  static func newestRelease(in versions: [String], releases: SwiftReleases) -> String {
-    let released = versions.filter { !VersionLabel($0).isNightly }
-    let newest = released.max { releases.ordered($0) < releases.ordered($1) }
-    return newest ?? versions.last ?? ""
-  }
 }
 
 /// The oldest toolchain the package builds on. A version below it cannot resolve the

tests/MatrixGeneratorValidator/Tests/MatrixGeneratorTests/CoreTests.swift

@@ -460,19 +460,23 @@ struct EmptiedByTheMinimumTests {
     #expect(!raised.entries.isEmpty)
   }
 
-  @Test("The default Cxx-interop version is filtered like any other")
-  func defaultedVersionIsChecked() throws {
-    // The check names no version: it defaults to the newest release in the Linux
-    // list, which a newer manifest puts below the minimum. This is what a caller
-    // bumping swift-tools-version ahead of the released toolchains hits, and the
-    // Linux and Windows nightlies survive to keep the matrix non-empty.
+  @Test("The Cxx interop check follows the Linux list and is filtered like it")
+  func cxxInteropFollowsTheLinuxList() throws {
+    // The check names no versions, so it runs on each version the Linux tests run, and a
+    // manifest that needs a newer toolchain removes the same ones from both.
     let generated = try Generator.run(
-      ["ENABLE_CXX_INTEROP": "true", "ENABLE_LINUX": "true", "ENABLE_WINDOWS": "true"],
-      manifests: ["Package.swift": Generator.manifest(toolsVersion: "6.5")]
+      [
+        "ENABLE_CXX_INTEROP": "true",
+        "ENABLE_LINUX": "true",
+        "LINUX_SWIFT_VERSIONS": #"["6.1","6.2","nightly-main"]"#,
+      ],
+      manifests: ["Package.swift": Generator.manifest(toolsVersion: "6.2")]
+    )
+    #expect(generated.exitCode == 0, "\(generated.standardError)")
+    #expect(
+      generated.names.filter { $0.hasPrefix("Cxx interop") }
+        == ["Cxx interop Swift 6.2", "Cxx interop Swift nightly-main"]
     )
-    #expect(generated.exitCode != 0)
-    #expect(generated.standardError.contains("enable_cxx_interop"), "\(generated.standardError)")
-    #expect(generated.standardError.contains("current"), "\(generated.standardError)")
   }
 
   @Test(

tests/MatrixGeneratorValidator/Tests/MatrixGeneratorTests/EntryPointTests.swift

@@ -57,7 +57,11 @@ struct EntryPointDefaultTests {
           "Android SDK Swift current NDK r30",
           "Android SDK Swift nightly-release NDK r30",
           "Android SDK Swift nightly-main NDK r30",
+          "Cxx interop Swift current-2",
+          "Cxx interop Swift current-1",
           "Cxx interop Swift current",
+          "Cxx interop Swift nightly-release",
+          "Cxx interop Swift nightly-main",
         ]
       ),
       (

tests/MatrixGeneratorValidator/Tests/MatrixGeneratorTests/JobGroupTests.swift

@@ -92,15 +92,14 @@ struct SDKTests {
 
 @Suite("Cxx interop")
 struct SupplementaryCheckTests {
-  @Test("Cxx interop defaults to one version and can enter a subdirectory")
+  @Test("Cxx interop defaults to the Linux versions and can enter a subdirectory")
   func cxxInteropScope() throws {
     let generated = try Generator.run([
       "ENABLE_CXX_INTEROP": "true",
       "LINUX_SWIFT_VERSIONS": #"["6.2","6.3"]"#,
       "LINUX_SETUP_COMMAND": "cd sub",
     ])
-    #expect(generated.count == 1)
-    #expect(generated.entries.first?.swiftBuild?.swiftVersion == "6.3")
+    #expect(generated.entries.compactMap { $0.swiftBuild?.swiftVersion } == ["6.2", "6.3"])
     // check-cxx-interop.sh reads the manifest in the working directory.
     #expect(generated.entries.first?.setupCommand == "cd sub")
     #expect(generated.entries.first?.command == "${SCRIPTS_ROOT}/check-cxx-interop.sh")