Fix availability logic refactor

Sources/SwiftDocC/Infrastructure/Workspace/DefaultAvailability.swift

@@ -1,7 +1,7 @@
 /*
  This source file is part of the Swift.org open source project
 
- Copyright (c) 2021-2024 Apple Inc. and the Swift project authors
+ Copyright (c) 2021-2025 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
@@ -131,28 +131,30 @@ public struct DefaultAvailability: Codable, Equatable {
 
     /// Fallback availability information for platforms we either don't emit SGFs for
     /// or have the same availability information as another platform.
-    package static let fallbackPlatforms: [PlatformName: PlatformName] = [
-        .catalyst: .iOS,
-        .iPadOS: .iOS,
-    ]
+    ///
+    /// The key corresponds to the fallback platform and the value to the platform it's
+    /// fallbacking from.
+    package static let fallbackPlatforms: [PlatformName: [PlatformName]] = [.iOS: [.catalyst, .iPadOS]]
 
     /// Creates a default availability module.
     /// - Parameter modules: A map of modules and the default platform availability for symbols in that module.
     public init(with modules: [String: [ModuleAvailability]]) {
             self.modules = modules.mapValues { platformAvailabilities -> [DefaultAvailability.ModuleAvailability] in
             // If a module doesn't contain default availability information for any of the fallback platforms,
             // infer it from the corresponding mapped value.
-            platformAvailabilities + DefaultAvailability.fallbackPlatforms.compactMap { (platform, fallbackPlatform) in
-                if !platformAvailabilities.contains(where: { $0.platformName == platform }),
-                   let fallbackAvailability = platformAvailabilities.first(where: { $0.platformName == fallbackPlatform }),
-                   let fallbackIntroducedVersion = fallbackAvailability.introducedVersion
-                {
-                    return DefaultAvailability.ModuleAvailability(
-                        platformName: platform,
-                        platformVersion: fallbackIntroducedVersion
-                    )
+            platformAvailabilities + DefaultAvailability.fallbackPlatforms.flatMap { (fallbackPlatform, platform) in
+                platform.compactMap { platformName in
+                    if !platformAvailabilities.contains(where: { $0.platformName == platformName }),
+                       let fallbackAvailability = platformAvailabilities.first(where: { $0.platformName == fallbackPlatform }),
+                       let fallbackIntroducedVersion = fallbackAvailability.introducedVersion
+                    {
+                        return DefaultAvailability.ModuleAvailability(
+                            platformName: platformName,
+                            platformVersion: fallbackIntroducedVersion
+                        )
+                    }
+                    return nil
                 }
-                return nil
             }
         }
     }

Sources/SwiftDocC/Semantics/Symbol/Symbol.swift

@@ -1,7 +1,7 @@
 /*
  This source file is part of the Swift.org open source project
 
- Copyright (c) 2021-2024 Apple Inc. and the Swift project authors
+ Copyright (c) 2021-2025 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
@@ -591,10 +591,12 @@ extension Symbol {
     func mergeAvailabilities(unifiedSymbol: UnifiedSymbolGraph.Symbol) {
         for (selector, mixins) in unifiedSymbol.mixins {
             let trait = DocumentationDataVariantsTrait(for: selector)
-            if let unifiedSymbolAvailability = mixins[SymbolGraph.Symbol.Availability.mixinKey] as? SymbolGraph.Symbol.Availability {
+            guard let availabilityVariantTrait = availabilityVariants[trait] else {
+                return
+            }
+            if let unifiedSymbolAvailability = mixins.getValueIfPresent(for: SymbolGraph.Symbol.Availability.self) {
                 unifiedSymbolAvailability.availability.forEach { availabilityItem in
-                    guard let availabilityVariantTrait = availabilityVariants[trait] else { return }
-                    if (availabilityVariantTrait.availability.contains(where: { $0.domain?.rawValue == availabilityItem.domain?.rawValue })) {
+                    guard availabilityVariantTrait.availability.firstIndex(where: { $0.domain?.rawValue == availabilityItem.domain?.rawValue }) == nil else {
                         return
                     }
                     availabilityVariants[trait]?.availability.append(availabilityItem)

Sources/SwiftDocCTestUtilities/FilesAndFolders.swift

@@ -112,7 +112,7 @@ public struct InfoPlist: File, DataRepresentable {
             self.identifier = identifier
             self.defaultAvailability = defaultAvailability
         }
-
+        
         public init(from decoder: any Decoder) throws {
             let container = try decoder.container(keyedBy: DocumentationBundle.Info.CodingKeys.self)
             displayName = try container.decodeIfPresent(String.self, forKey: .displayName)

Sources/SwiftDocCUtilities/Action/Actions/Convert/ConvertAction.swift

@@ -1,7 +1,7 @@
 /*
  This source file is part of the Swift.org open source project
 
- Copyright (c) 2021-2024 Apple Inc. and the Swift project authors
+ Copyright (c) 2021-2025 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
@@ -145,8 +145,10 @@ public struct ConvertAction: AsyncAction {
         // Inject current platform versions if provided
         if var currentPlatforms {
             // Add missing platforms if their fallback platform is present.
-            for (platform, fallbackPlatform) in DefaultAvailability.fallbackPlatforms where currentPlatforms[platform.displayName] == nil {
-                currentPlatforms[platform.displayName] = currentPlatforms[fallbackPlatform.displayName]
+            for (fallbackPlatform, platforms) in DefaultAvailability.fallbackPlatforms {
+                for platform in platforms where currentPlatforms[platform.displayName] == nil {
+                    currentPlatforms[platform.displayName] = currentPlatforms[fallbackPlatform.displayName]
+                }
             }
             configuration.externalMetadata.currentPlatforms = currentPlatforms
         }