Remove workaround for compiler bug breaking constrained existential types on iOS 15 release builds

Closes #178

Tusker/MultiThreadDictionary.swift

@@ -13,20 +13,24 @@ import os
 // to make the lock semantics more clear
 @available(iOS, obsoleted: 16.0)
 class MultiThreadDictionary<Key: Hashable & Sendable, Value: Sendable> {
-    private let lock: LockHolder<[AnyHashable: Any]>
+    private let lock: any Lock<[Key: Value]>
     
     init() {
-        self.lock = LockHolder(initialState: [:])
+        if #available(iOS 16.0, *) {
+            self.lock = OSAllocatedUnfairLock(initialState: [:])
+        } else {
+            self.lock = UnfairLock(initialState: [:])
+        }
     }
     
     subscript(key: Key) -> Value? {
         get {
-            return try! lock.withLock { dict in
+            return lock.withLock { dict in
                 dict[key]
-            } as! Value?
+            }
         }
         set(value) {
-            _ = try! lock.withLock { dict in
+            _ = lock.withLock { dict in
                 dict[key] = value
             }
         }
@@ -34,40 +38,21 @@ class MultiThreadDictionary<Key: Hashable & Sendable, Value: Sendable> {
     
     /// If the result of this function is unused, it is preferable to use `removeValueWithoutReturning` as it executes asynchronously and doesn't block the calling thread.
     func removeValue(forKey key: Key) -> Value? {
-        return try! lock.withLock { dict in
+        return lock.withLock { dict in
             dict.removeValue(forKey: key)
-        } as! Value?
+        }
     }
     
     func contains(key: Key) -> Bool {
-        return try! lock.withLock { dict in
+        return lock.withLock { dict in
             dict.keys.contains(key)
-        } as! Bool
+        }
     }
     
     // TODO: this should really be throws/rethrows but the stupid type-erased lock makes that not possible
-    func withLock<R>(_ body: @Sendable (inout [Key: Value]) -> R) -> R where R: Sendable {
-        return try! lock.withLock { dict in
-            var downcasted = dict as! [Key: Value]
-            defer { dict = downcasted }
-            return body(&downcasted)
-        } as! R
-    }
-}
-
-// this type erased struct is necessary due to a compiler bug with stored constrained existential types
-// see https://github.com/apple/swift/issues/61403
-// see #178
-fileprivate struct LockHolder<State> {
-    let withLock: (_ body: @Sendable (inout State) throws -> any Sendable) throws -> any Sendable
-    
-    init(initialState: State) {
-        if #available(iOS 16.0, *) {
-            let lock = OSAllocatedUnfairLock(initialState: initialState)
-            self.withLock = lock.withLock(_:)
-        } else {
-            let lock = UnfairLock(initialState: initialState)
-            self.withLock = lock.withLock(_:)
+    func withLock<R>(_ body: @Sendable (inout [Key: Value]) throws -> R) rethrows -> R where R: Sendable {
+        return try lock.withLock { dict in
+            return try body(&dict)
         }
     }
 }