diff --git a/IntegrationTests/Package.swift b/IntegrationTests/Package.swift
index 05b73b3..e72f1af 100644
--- a/IntegrationTests/Package.swift
+++ b/IntegrationTests/Package.swift
@@ -30,6 +30,7 @@ let package = Package(
                 .copy("Fixtures/MixedTargets"),
                 .copy("Fixtures/TargetWithDocCCatalog"),
                 .copy("Fixtures/PackageWithSnippets"),
+                .copy("Fixtures/LibraryTargetWithExtensionSymbols"),
             ]
         ),
     ]
diff --git a/IntegrationTests/Tests/Fixtures/LibraryTargetWithExtensionSymbols/Package.swift b/IntegrationTests/Tests/Fixtures/LibraryTargetWithExtensionSymbols/Package.swift
new file mode 100644
index 0000000..a3efff1
--- /dev/null
+++ b/IntegrationTests/Tests/Fixtures/LibraryTargetWithExtensionSymbols/Package.swift
@@ -0,0 +1,30 @@
+// swift-tools-version: 5.6
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2023 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 Swift project authors
+
+import Foundation
+import PackageDescription
+
+let package = Package(
+    name: "LibraryTargetWithExtensionSymbols",
+    targets: [
+        .target(name: "Library"),
+    ]
+)
+
+// We only expect 'swift-docc-plugin' to be a sibling when this package
+// is running as part of a test.
+//
+// This allows the package to compile outside of tests for easier
+// test development.
+if FileManager.default.fileExists(atPath: "../swift-docc-plugin") {
+    package.dependencies += [
+        .package(path: "../swift-docc-plugin"),
+    ]
+}
diff --git a/IntegrationTests/Tests/Fixtures/LibraryTargetWithExtensionSymbols/Sources/Library/Library.swift b/IntegrationTests/Tests/Fixtures/LibraryTargetWithExtensionSymbols/Sources/Library/Library.swift
new file mode 100644
index 0000000..62ca5ca
--- /dev/null
+++ b/IntegrationTests/Tests/Fixtures/LibraryTargetWithExtensionSymbols/Sources/Library/Library.swift
@@ -0,0 +1,62 @@
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2023 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 Swift project authors
+
+/// This is foo's documentation.
+///
+/// Foo is a public struct and should be included in documentation.
+public struct Foo {
+    public func foo() {}
+}
+
+/// This is the documentation for ``Swift/Array``.
+///
+/// This is the extension to ``Array`` with the longest documentation
+/// comment, thus it is used for doucmenting the extended type in this
+/// target.
+extension Array {
+    /// This is the documentation for the ``isArray`` property
+    /// we added to ``Swift/Array``.
+    ///
+    /// This is a public extension to an external type and should be included
+    /// in the documentation.
+    public var isArray: Bool { true }
+}
+
+/// This is the documentation for ``Swift/Int``.
+///
+/// This is the extension to ``Int`` with the longest documentation
+/// comment, thus it is used for doucmenting the extended type in this
+/// target.
+extension Int {
+    /// This is the documentation for the ``isArray`` property
+    /// we added to ``Swift/Int``.
+    ///
+    /// This is a public extension to an external type and should be included
+    /// in the documentation.
+    public var isArray: Bool { false }
+}
+
+
+/// This is the documentation for ``CustomFooConvertible``.
+///
+/// This is a public protocol and should be included in the documentation.
+public protocol CustomFooConvertible {
+    /// This is the documentation for ``CustomFooConvertible/asFoo``.
+    ///
+    /// This is a public protocol requirement and should be included in the documentation.
+    var asFoo: Foo { get }
+}
+
+/// This is not used as the documentation comment for ``Swift/Int``
+/// as it is shorter than the comment on the other extension to `Int`.
+extension Int: CustomFooConvertible {
+    /// This is the documentation for ``Swift/Int/asFoo``.
+    ///
+    /// This is a public protocol requirement implementation and should be included in the documentation.
+    public var asFoo: Foo { Foo() }
+}
diff --git a/IntegrationTests/Tests/TargetWithSwiftExtensionsTests.swift b/IntegrationTests/Tests/TargetWithSwiftExtensionsTests.swift
new file mode 100644
index 0000000..9993b75
--- /dev/null
+++ b/IntegrationTests/Tests/TargetWithSwiftExtensionsTests.swift
@@ -0,0 +1,91 @@
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2023 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 Swift project authors
+
+import Foundation
+import XCTest
+
+final class TargetWithSwiftExtensionsTests: ConcurrencyRequiringTestCase {
+#if swift(>=5.8)
+    let supportsIncludingSwiftExtendedTypes = true
+#else
+    let supportsIncludingSwiftExtendedTypes = false
+#endif
+    
+    override func setUpWithError() throws {
+        try XCTSkipUnless(
+            supportsIncludingSwiftExtendedTypes,
+            "The current toolchain does not support symbol graph generation for extended types."
+        )
+        
+        try super.setUpWithError()
+    }
+    
+    func testGenerateDocumentationWithoutEnablementFlag() throws {
+        let result = try swiftPackage(
+            "generate-documentation",
+            workingDirectory: try setupTemporaryDirectoryForFixture(named: "LibraryTargetWithExtensionSymbols")
+        )
+        
+        result.assertExitStatusEquals(0)
+        XCTAssertEqual(result.referencedDocCArchives.count, 1)
+        
+        let doccArchiveURL = try XCTUnwrap(result.referencedDocCArchives.first)
+        
+        let dataDirectoryContents = try filesIn(.dataSubdirectory, of: doccArchiveURL)
+        
+        XCTAssertEqual(
+            Set(dataDirectoryContents.map(\.lastTwoPathComponents)),
+            [
+                "documentation/library.json",
+                
+                "library/foo.json",
+                "foo/foo().json",
+                
+                "library/customfooconvertible.json",
+                "customfooconvertible/asfoo.json",
+            ]
+        )
+    }
+    
+    func testGenerateDocumentationWithEnablementFlag() throws {
+        let result = try swiftPackage(
+            "generate-documentation",
+            "--include-extended-types",
+            workingDirectory: try setupTemporaryDirectoryForFixture(named: "LibraryTargetWithExtensionSymbols")
+        )
+        
+        result.assertExitStatusEquals(0)
+        XCTAssertEqual(result.referencedDocCArchives.count, 1)
+        
+        let doccArchiveURL = try XCTUnwrap(result.referencedDocCArchives.first)
+        
+        let dataDirectoryContents = try filesIn(.dataSubdirectory, of: doccArchiveURL)
+        
+        XCTAssertEqual(
+            Set(dataDirectoryContents.map(\.lastTwoPathComponents)),
+            [
+                "documentation/library.json",
+                "library/swift.json",
+                
+                "swift/int.json",
+                "int/isarray.json",
+                "int/asfoo.json",
+                "int/customfooconvertible-implementations.json",
+                
+                "swift/array.json",
+                "array/isarray.json",
+                
+                "library/foo.json",
+                "foo/foo().json",
+                
+                "library/customfooconvertible.json",
+                "customfooconvertible/asfoo.json",
+            ]
+        )
+    }
+}
diff --git a/Plugins/SharedPackagePluginExtensions/PackageManager+getSymbolGraphsForDocC.swift b/Plugins/SharedPackagePluginExtensions/PackageManager+getSymbolGraphsForDocC.swift
index 24c59d1..154b8c2 100644
--- a/Plugins/SharedPackagePluginExtensions/PackageManager+getSymbolGraphsForDocC.swift
+++ b/Plugins/SharedPackagePluginExtensions/PackageManager+getSymbolGraphsForDocC.swift
@@ -38,12 +38,27 @@ extension PackageManager {
         for target: SwiftSourceModuleTarget,
         context: PluginContext,
         verbose: Bool,
-        snippetExtractor: SnippetExtractor?
+        snippetExtractor: SnippetExtractor?,
+        customSymbolGraphOptions: [PluginFlag]
     ) throws -> DocCSymbolGraphResult {
         // First generate the primary symbol graphs containing information about the
         // symbols defined in the target itself.
         
-        let symbolGraphOptions = target.defaultSymbolGraphOptions(in: context.package)
+        var symbolGraphOptions = target.defaultSymbolGraphOptions(in: context.package)
+
+        // Modify the symbol graph options with the custom ones
+        for customSymbolGraphOption in customSymbolGraphOptions {
+            switch customSymbolGraphOption {
+            case .extendedTypes:
+#if swift(>=5.8)
+                symbolGraphOptions.emitExtensionBlocks = true
+#else
+                print("warning: detected '--include-extended-types' option, which is incompatible with your swift version (required: 5.8)")
+#endif
+            default:
+                fatalError("error: unknown PluginFlag (\(customSymbolGraphOption.parsedValues.joined(separator: ", "))) detected in symbol graph generation - please create an issue at https://github.com/apple/swift-docc-plugin")
+         }
+        }
         
         if verbose {
             print("symbol graph options: '\(symbolGraphOptions)'")
diff --git a/Plugins/SharedPackagePluginExtensions/Target+defaultSymbolGraphOptions.swift b/Plugins/SharedPackagePluginExtensions/Target+defaultSymbolGraphOptions.swift
index 724c917..9b00cbe 100644
--- a/Plugins/SharedPackagePluginExtensions/Target+defaultSymbolGraphOptions.swift
+++ b/Plugins/SharedPackagePluginExtensions/Target+defaultSymbolGraphOptions.swift
@@ -27,7 +27,21 @@ extension SwiftSourceModuleTarget {
         return PackageManager.SymbolGraphOptions(
             minimumAccessLevel: targetMinimumAccessLevel,
             includeSynthesized: true,
-            includeSPI: false
+            includeSPI: false,
+            emitExtensionBlocks: false
         )
     }
 }
+
+
+#if swift(<5.8)
+private extension PackageManager.SymbolGraphOptions {
+    /// A compatibility layer for lower Swift versions which discards unknown parameters.
+    init(minimumAccessLevel: PackagePlugin.PackageManager.SymbolGraphOptions.AccessLevel = .public,
+         includeSynthesized: Bool = false,
+         includeSPI: Bool = false,
+         emitExtensionBlocks: Bool) {
+        self.init(minimumAccessLevel: minimumAccessLevel, includeSynthesized: includeSynthesized, includeSPI: includeSPI)
+    }
+}
+#endif
diff --git a/Plugins/Swift-DocC Convert/SwiftDocCConvert.swift b/Plugins/Swift-DocC Convert/SwiftDocCConvert.swift
index 2d5178f..bb91b71 100644
--- a/Plugins/Swift-DocC Convert/SwiftDocCConvert.swift	
+++ b/Plugins/Swift-DocC Convert/SwiftDocCConvert.swift	
@@ -70,7 +70,8 @@ import PackagePlugin
                 for: target,
                 context: context,
                 verbose: verbose,
-                snippetExtractor: snippetExtractor
+                snippetExtractor: snippetExtractor,
+                customSymbolGraphOptions: parsedArguments.symbolGraphArguments
             )
             
             if try FileManager.default.contentsOfDirectory(atPath: symbolGraphs.targetSymbolGraphsDirectory.path).isEmpty {
diff --git a/Plugins/Swift-DocC Preview/SwiftDocCPreview.swift b/Plugins/Swift-DocC Preview/SwiftDocCPreview.swift
index 454f0ea..9bb82e6 100644
--- a/Plugins/Swift-DocC Preview/SwiftDocCPreview.swift	
+++ b/Plugins/Swift-DocC Preview/SwiftDocCPreview.swift	
@@ -83,7 +83,8 @@ import PackagePlugin
             for: target,
             context: context,
             verbose: verbose,
-            snippetExtractor: snippetExtractor
+            snippetExtractor: snippetExtractor,
+            customSymbolGraphOptions: parsedArguments.symbolGraphArguments
         )
         
         if try FileManager.default.contentsOfDirectory(atPath: symbolGraphs.targetSymbolGraphsDirectory.path).isEmpty {
diff --git a/Sources/SwiftDocCPluginUtilities/HelpInformation.swift b/Sources/SwiftDocCPluginUtilities/HelpInformation.swift
index 7c1ec62..77fb234 100644
--- a/Sources/SwiftDocCPluginUtilities/HelpInformation.swift
+++ b/Sources/SwiftDocCPluginUtilities/HelpInformation.swift
@@ -43,10 +43,17 @@ public enum HelpInformation {
             helpText = previewPluginHelpOverview
         }
         
-        let supportedPluginFlags = [
+        var supportedPluginFlags = [
             PluginFlag.disableIndex,
         ]
         
+        // stops 'not mutated' warning for Swift 5.7 and lower
+        supportedPluginFlags += []
+        
+#if swift(>=5.8)
+        supportedPluginFlags += [PluginFlag.extendedTypes]
+#endif
+        
         for flag in supportedPluginFlags {
             helpText += """
                   \(flag.parsedValues.sorted().joined(separator: ", "))
diff --git a/Sources/SwiftDocCPluginUtilities/ParsedArguments.swift b/Sources/SwiftDocCPluginUtilities/ParsedArguments.swift
index fcbbc1b..d3e7063 100644
--- a/Sources/SwiftDocCPluginUtilities/ParsedArguments.swift
+++ b/Sources/SwiftDocCPluginUtilities/ParsedArguments.swift
@@ -21,8 +21,8 @@ public struct ParsedArguments {
     
     /// Returns the arguments that should be passed to `docc` to invoke the given plugin action.
     ///
-    /// Merges the arguments provided upon initialization of the parsed arguments
-    /// with default fallback values for required options that were not provided.
+    /// Merges the arguments provided upon initialization of the parsed arguments that are relevant
+    /// to `docc` with default fallback values for required options that were not provided.
     ///
     /// For example, if ParsedArguments is initialized like so:
     ///
@@ -90,7 +90,7 @@ public struct ParsedArguments {
         symbolGraphDirectoryPath: String,
         outputPath: String
     ) -> Arguments {
-        var doccArguments = arguments
+        var doccArguments = arguments.filter(for: .docc)
         
         // Iterate through the flags required for the `docc` invocation
         // and append any that are not already present.
@@ -153,8 +153,19 @@ public struct ParsedArguments {
     /// Creates a new set of parsed arguments with the given arguments.
     public init(_ arguments: [String]) {
         self.arguments = arguments
+        
+        let symbolGraphArguments = arguments.filter(for: .dumpSymbolGraph)
+        
+        self.symbolGraphArguments = ParsedArguments.ArgumentConsumer.dumpSymbolGraph.flags.filter { option in
+            option.parsedValues.contains(where: symbolGraphArguments.contains)
+        }
     }
     
+    // Build array with plugin flags that modify the symbol graph generation,
+    // filtering from the available custom symbol graph options those
+    // that correspond to the received flags
+    var symbolGraphArguments: [PluginFlag]
+    
     /// The command-line options required by the `docc` tool.
     private static let requiredOptions: [CommandLineOption] = [
         .fallbackDisplayName,
@@ -170,6 +181,59 @@ public struct ParsedArguments {
     ]
     
     private static let argumentsTransformers: [ArgumentsTransforming] = [
-        PluginFlag.disableIndex
+        PluginFlag.disableIndex,
+        PluginFlag.extendedTypes
     ]
 }
+
+private extension ParsedArguments {
+    enum ArgumentConsumer: CaseIterable {
+        /// The `docc` command
+        case docc
+        /// The `swift package dump-symbol-graph` command
+        case dumpSymbolGraph
+        
+        /// Returns the flags applicable to an `ArgumentConsumer`.
+        ///
+        /// If `flags.isEmpty` is `true`, this `ArgumentConsumer` is assumed to
+        /// consume all flags not consumed by any of the other `ArgumentConsumer`s.
+        var flags: [PluginFlag] {
+            switch self {
+            case .dumpSymbolGraph:
+                return [
+                    PluginFlag.extendedTypes
+                ]
+            case .docc:
+                return []
+            }
+        }
+    }
+}
+
+private extension Arguments {
+    /// Returns the subset of arguments which are applicable to the given `consumer`.
+    func filter(for consumer: ParsedArguments.ArgumentConsumer) -> Arguments {
+        if !consumer.flags.isEmpty {
+            // If the consumer can provide a complete list of valid flags,
+            // we only include elements that are included in one of these flags'
+            // `parsedValues`, i.e. if one of these flags can be applied to the
+            // element.
+            let flagsToInclude = consumer.flags
+            return self.filter { argument in
+                flagsToInclude.contains(where: { flag in
+                    flag.parsedValues.contains(argument)
+                })
+            }
+        } else {
+            // If the consumer cannot provide a complete list of valid flags, (which
+            // should only happen for the `.docc` case) we return all elements
+            // that are not applicable to any of the other `ArgumentConsumer`s.
+            let flagsToExclude = ParsedArguments.ArgumentConsumer.allCases.flatMap(\.flags)
+            return self.filter { argument in
+                !flagsToExclude.contains(where: { flag in
+                    flag.parsedValues.contains(argument)
+                })
+            }
+        }
+    }
+}
diff --git a/Sources/SwiftDocCPluginUtilities/PluginFlags/ExtendedTypesFlag.swift b/Sources/SwiftDocCPluginUtilities/PluginFlags/ExtendedTypesFlag.swift
new file mode 100644
index 0000000..d22c623
--- /dev/null
+++ b/Sources/SwiftDocCPluginUtilities/PluginFlags/ExtendedTypesFlag.swift
@@ -0,0 +1,29 @@
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2022 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 Swift project authors
+
+extension PluginFlag {
+    /// Include extended types in documentation archives.
+    ///
+    /// Enables the extension block symbol format when calling the
+    /// dump symbol graph API.
+    ///
+    /// - Note: This flag is only available starting from Swift 5.8. It should
+    /// be hidden from the `--help` command for lower toolchain versions.
+    /// However, we do not hide the flag entirely, because this enables us to give
+    /// a more precise warning when accidentally used with Swift 5.7 or lower.
+    static let extendedTypes = PluginFlag(
+        parsedValues: [
+            "--include-extended-types",
+        ],
+        abstract: "Include extended types from other modules in the produced DocC archive.",
+        description: """
+            Allows documenting symbols that a target adds to its dependencies.
+            """,
+        argumentTransformation: { $0 }
+    )
+}
diff --git a/Sources/SwiftDocCPluginUtilities/PluginFlags/PluginFlag+Equatable.swift b/Sources/SwiftDocCPluginUtilities/PluginFlags/PluginFlag+Equatable.swift
new file mode 100644
index 0000000..214c336
--- /dev/null
+++ b/Sources/SwiftDocCPluginUtilities/PluginFlags/PluginFlag+Equatable.swift
@@ -0,0 +1,13 @@
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2022 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 Swift project authors
+
+extension PluginFlag: Equatable {
+    static func ==(lhs: PluginFlag, rhs: PluginFlag) -> Bool {
+        return lhs.parsedValues == rhs.parsedValues
+    }
+}
diff --git a/Tests/SwiftDocCPluginUtilitiesTests/HelpInformationTests.swift b/Tests/SwiftDocCPluginUtilitiesTests/HelpInformationTests.swift
index 17dbfb2..2175a25 100644
--- a/Tests/SwiftDocCPluginUtilitiesTests/HelpInformationTests.swift
+++ b/Tests/SwiftDocCPluginUtilitiesTests/HelpInformationTests.swift
@@ -39,7 +39,7 @@ final class HelpInformationTests: XCTestCase {
               --product <product>     Generate documentation for the specified product.
               --disable-indexing, --no-indexing
                                       Disable indexing for the produced DocC archive.
-                    Produces a DocC archive that is best-suited for hosting online but incompatible with Xcode.
+                    Produces a DocC archive that is best-suited for hosting online but incompatible with Xcode.\(includeExtendedTypesSection)
 
             DOCC OPTIONS:
               --platform <platform>   Set the current release version of a platform.
@@ -128,7 +128,7 @@ final class HelpInformationTests: XCTestCase {
               --product <product>     Preview documentation for the specified product.
               --disable-indexing, --no-indexing
                                       Disable indexing for the produced DocC archive.
-                    Produces a DocC archive that is best-suited for hosting online but incompatible with Xcode.
+                    Produces a DocC archive that is best-suited for hosting online but incompatible with Xcode.\(includeExtendedTypesSection)
             
             DOCC OPTIONS:
               --platform <platform>   Set the current release version of a platform.
@@ -184,3 +184,13 @@ final class HelpInformationTests: XCTestCase {
     }
 }
 
+#if swift(>=5.8)
+private let includeExtendedTypesSection = """
+
+  --include-extended-types
+                          Include extended types from other modules in the produced DocC archive.
+        Allows documenting symbols that a target adds to its dependencies.
+"""
+#else
+private let includeExtendedTypesSection = ""
+#endif
diff --git a/Tests/SwiftDocCPluginUtilitiesTests/ParsedArgumentsTests.swift b/Tests/SwiftDocCPluginUtilitiesTests/ParsedArgumentsTests.swift
index ccc9146..e476a19 100644
--- a/Tests/SwiftDocCPluginUtilitiesTests/ParsedArgumentsTests.swift
+++ b/Tests/SwiftDocCPluginUtilitiesTests/ParsedArgumentsTests.swift
@@ -442,4 +442,32 @@ final class ParsedArgumentsTests: XCTestCase {
             ]
         )
     }
+    
+    func testDocCArgumentsWithDumpSymbolGraphArguments() {
+        let dumpSymbolGraphArguments = ParsedArguments(["--include-extended-types"])
+        
+        let doccArguments = dumpSymbolGraphArguments.doccArguments(
+            action: .convert,
+            targetKind: .executable,
+            doccCatalogPath: "/my/catalog.docc",
+            targetName: "MyTarget",
+            symbolGraphDirectoryPath: "/my/symbol-graph",
+            outputPath: "/my/output-path"
+        )
+        
+        XCTAssertFalse(doccArguments.contains("--include-extended-types"))
+    }
+    
+    func testDumpSymbolGraphArguments() {
+        let dumpSymbolGraphArguments = ParsedArguments(["--include-extended-types"])
+        
+        XCTAssertEqual(dumpSymbolGraphArguments.symbolGraphArguments, [.extendedTypes])
+    }
+    
+    func testDumpSymbolGraphArgumentsWithDocCArguments() {
+        let dumpSymbolGraphArguments = ParsedArguments(["--fallback-default-module-kind", "Executable"])
+        
+        
+        XCTAssertEqual(dumpSymbolGraphArguments.symbolGraphArguments, [])
+    }
 }
diff --git a/Tests/SwiftDocCPluginUtilitiesTests/PluginFlags/ExtendedTypesFlagTests.swift b/Tests/SwiftDocCPluginUtilitiesTests/PluginFlags/ExtendedTypesFlagTests.swift
new file mode 100644
index 0000000..c45be20
--- /dev/null
+++ b/Tests/SwiftDocCPluginUtilitiesTests/PluginFlags/ExtendedTypesFlagTests.swift
@@ -0,0 +1,31 @@
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2022 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 Swift project authors
+
+import Foundation
+@testable import SwiftDocCPluginUtilities
+import XCTest
+
+final class ExtendedTypesFlagTests: XCTestCase {
+    func testReplacesIncludeExtendedTypesFlagWhenPresent() {
+        XCTAssertEqual(
+            PluginFlag.extendedTypes.transform(
+                ["--include-extended-types", "--other-flag"]
+            ),
+            ["--other-flag"]
+        )
+    }
+    
+    func testDoesNotAddEmitExtensionBlockSymbolsFlagWhenAbsent() {
+        XCTAssertEqual(
+            PluginFlag.extendedTypes.transform(
+                ["--other-flag"]
+            ),
+            ["--other-flag"]
+        )
+    }
+}
diff --git a/bin/check-source b/bin/check-source
index 5d1dc64..5205f28 100755
--- a/bin/check-source
+++ b/bin/check-source
@@ -18,7 +18,7 @@ here="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
 
 function replace_acceptable_years() {
     # this needs to replace all acceptable forms with 'YEARS'
-    sed -e 's/20[12][789012]-20[12][89012]/YEARS/' -e 's/20[12][89012]/YEARS/'
+    sed -e 's/20[12][7890123]-20[12][890123]/YEARS/' -e 's/20[12][890123]/YEARS/'
 }
 
 printf "=> Checking for unacceptable language… "