Fix false positive for subscript(dynamicMember:) in external type extensions

Sources/SourceGraph/Mutators/DynamicMemberRetainer.swift

@@ -10,8 +10,11 @@ final class DynamicMemberRetainer: SourceGraphMutator {
     }
 
     func mutate() throws {
+        // Retain all subscript(dynamicMember:) declarations. This signature is specific to
+        // @dynamicMemberLookup and may be declared in extensions of external types where we
+        // cannot verify the attribute exists.
         for decl in graph.declarations(ofKind: .functionSubscript) {
-            if decl.name == "subscript(dynamicMember:)", decl.parent?.attributes.contains(where: { $0.name == "dynamicMemberLookup" }) ?? false {
+            if decl.name == "subscript(dynamicMember:)" {
                 graph.markRetained(decl)
             }
         }

Tests/Fixtures/Sources/RetentionFixtures/testRetainsDynamicMemberLookupSubscriptInExternalTypeExtension.swift

@@ -0,0 +1,21 @@
+import Foundation
+
+// Extension on external @dynamicMemberLookup type (AttributeDynamicLookup)
+public extension AttributeDynamicLookup {
+    subscript<T: AttributedStringKey>(
+        dynamicMember keyPath: KeyPath<AttributeScopes.FixtureAttributes, T>
+    ) -> T {
+        self[T.self]
+    }
+}
+
+public extension AttributeScopes {
+    struct FixtureAttributes: AttributeScope {
+        let myAttribute: MyFixtureAttribute
+    }
+}
+
+public enum MyFixtureAttribute: AttributedStringKey {
+    public typealias Value = String
+    public static let name = "MyFixtureAttribute"
+}

Tests/PeripheryTests/RetentionTest.swift

@@ -946,6 +946,14 @@ final class RetentionTest: FixtureSourceGraphTestCase {
         }
     }
 
+    func testRetainsDynamicMemberLookupSubscriptInExternalTypeExtension() {
+        analyze(retainPublic: true) {
+            assertReferenced(.extensionEnum("AttributeDynamicLookup")) {
+                self.assertReferenced(.functionSubscript("subscript(dynamicMember:)"))
+            }
+        }
+    }
+
     func testRetainsCodableProperties() {
         analyze(
             retainPublic: true,

Tests/Shared/DeclarationDescription.swift

@@ -105,4 +105,8 @@ struct DeclarationDescription: CustomStringConvertible {
     static func extensionClass(_ name: String, line: Int? = nil) -> Self {
         self.init(kind: .extensionClass, name: name, line: line)
     }
+
+    static func extensionEnum(_ name: String, line: Int? = nil) -> Self {
+        self.init(kind: .extensionEnum, name: name, line: line)
+    }
 }