Fix the issue where AtomEffectBuilder fails to demangle witness for the return type of the builder with multiple blocks in iOS 16 (#193)

Author: ra1028

Sources/Atoms/Effect/AtomEffectBuilder.swift

@@ -19,17 +19,19 @@
 @MainActor
 @resultBuilder
 public enum AtomEffectBuilder {
+    public static func buildBlock() -> some AtomEffect {
+        EmptyEffect()
+    }
+
     public static func buildBlock<Effect: AtomEffect>(_ effect: Effect) -> Effect {
         effect
     }
 
-    public static func buildBlock<each Effect: AtomEffect>(_ effect: repeat each Effect) -> some AtomEffect {
-        if #available(iOS 17, macOS 14, tvOS 17, watchOS 10, *) {
-            return BlockEffect(repeat each effect)
-        }
-        else {
-            return BlockBCEffect(repeat each effect)
-        }
+    // NB: The combination of variadic type and opaque return type in iOS 16 fails to demangle
+    // the witness for the return type, which causes a runtime crash.
+    // Once iOS 16 support is dropped, this function will be changed to return an opaque type.
+    public static func buildBlock<each Effect: AtomEffect>(_ effect: repeat each Effect) -> BlockEffect {
+        BlockEffect(repeat each effect)
     }
 
     public static func buildIf<Effect: AtomEffect>(_ effect: Effect?) -> some AtomEffect {
@@ -54,6 +56,46 @@ public enum AtomEffectBuilder {
 }
 
 public extension AtomEffectBuilder {
+    // Use type pack once it is available in iOS 17 or newer.
+    // struct BlockEffect<each Effect: AtomEffect>: AtomEffect
+    struct BlockEffect: AtomEffect {
+        private let _initializing: @MainActor (Context) -> Void
+        private let _initialized: @MainActor (Context) -> Void
+        private let _updated: @MainActor (Context) -> Void
+        private let _released: @MainActor (Context) -> Void
+
+        internal init<each Effect: AtomEffect>(_ effect: repeat each Effect) {
+            _initializing = { context in
+                repeat (each effect).initializing(context: context)
+            }
+            _initialized = { context in
+                repeat (each effect).initialized(context: context)
+            }
+            _updated = { context in
+                repeat (each effect).updated(context: context)
+            }
+            _released = { context in
+                repeat (each effect).released(context: context)
+            }
+        }
+
+        public func initializing(context: Context) {
+            _initializing(context)
+        }
+
+        public func initialized(context: Context) {
+            _initialized(context)
+        }
+
+        public func updated(context: Context) {
+            _updated(context)
+        }
+
+        public func released(context: Context) {
+            _released(context)
+        }
+    }
+
     struct ConditionalEffect<TrueEffect: AtomEffect, FalseEffect: AtomEffect>: AtomEffect {
         internal enum Storage {
             case trueEffect(TrueEffect)
@@ -108,68 +150,7 @@ public extension AtomEffectBuilder {
 }
 
 private extension AtomEffectBuilder {
-    @available(iOS 17, macOS 14, tvOS 17, watchOS 10, *)
-    struct BlockEffect<each Effect: AtomEffect>: AtomEffect {
-        private let effect: (repeat each Effect)
-
-        init(_ effect: repeat each Effect) {
-            self.effect = (repeat each effect)
-        }
-
-        func initializing(context: Context) {
-            repeat (each effect).initializing(context: context)
-        }
-
-        func initialized(context: Context) {
-            repeat (each effect).initialized(context: context)
-        }
-
-        func updated(context: Context) {
-            repeat (each effect).updated(context: context)
-        }
-
-        func released(context: Context) {
-            repeat (each effect).released(context: context)
-        }
-    }
-
-    struct BlockBCEffect: AtomEffect {
-        private let _initializing: @MainActor (Context) -> Void
-        private let _initialized: @MainActor (Context) -> Void
-        private let _updated: @MainActor (Context) -> Void
-        private let _released: @MainActor (Context) -> Void
-
-        init<each Effect: AtomEffect>(_ effect: repeat each Effect) {
-            _initializing = { context in
-                repeat (each effect).initializing(context: context)
-            }
-            _initialized = { context in
-                repeat (each effect).initialized(context: context)
-            }
-            _updated = { context in
-                repeat (each effect).updated(context: context)
-            }
-            _released = { context in
-                repeat (each effect).released(context: context)
-            }
-        }
-
-        func initializing(context: Context) {
-            _initializing(context)
-        }
-
-        func initialized(context: Context) {
-            _initialized(context)
-        }
-
-        func updated(context: Context) {
-            _updated(context)
-        }
-
-        func released(context: Context) {
-            _released(context)
-        }
-    }
+    struct EmptyEffect: AtomEffect {}
 
     struct IfEffect<Effect: AtomEffect>: AtomEffect {
         private let effect: Effect?