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..e900fc8
--- /dev/null
+++ b/IntegrationTests/Tests/Fixtures/LibraryTargetWithExtensionSymbols/Sources/Library/Library.swift
@@ -0,0 +1,22 @@
+// 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() {}
+}
+
+extension Array {
+    public var isArray: Bool { true }
+}
+
+extension Int {
+    public var isArray: Bool { false }
+}
diff --git a/IntegrationTests/Tests/TargetWithSwiftExtensionsTests.swift b/IntegrationTests/Tests/TargetWithSwiftExtensionsTests.swift
new file mode 100644
index 0000000..59e6f3b
--- /dev/null
+++ b/IntegrationTests/Tests/TargetWithSwiftExtensionsTests.swift
@@ -0,0 +1,83 @@
+// 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
+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",
+            ]
+        )
+    }
+    
+    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",
+                
+                "swift/array.json",
+                "array/isarray.json",
+                
+                "library/foo.json",
+                "foo/foo().json",
+            ]
+        )
+    }
+}