diff --git a/Sources/MockoloFramework/Utils/StringExtensions.swift b/Sources/MockoloFramework/Utils/StringExtensions.swift
index 3e24050f..5a0025b6 100644
--- a/Sources/MockoloFramework/Utils/StringExtensions.swift
+++ b/Sources/MockoloFramework/Utils/StringExtensions.swift
@@ -119,6 +119,7 @@ extension String {
     static let arrayTypeSugarName = "[]"
     static let dictionaryTypeSugarName = "[:]"
     static let optionalTypeSugarName = "?"
+    static let optionalAutounwrappedTypeSugarName = "!"
 
     var safeName: String {
         var text = self
diff --git a/Sources/MockoloFramework/Utils/SwiftType.swift b/Sources/MockoloFramework/Utils/SwiftType.swift
index d1165cb2..3de21ced 100644
--- a/Sources/MockoloFramework/Utils/SwiftType.swift
+++ b/Sources/MockoloFramework/Utils/SwiftType.swift
@@ -57,7 +57,6 @@ struct SwiftType: Equatable, CustomStringConvertible {
     var kind: Kind
     var attributes: [String] = []
     var someOrAny: SomeOrAny?
-    var isIUO: Bool = false
     var hasEllipsis: Bool = false
 
     var typeName: String {
@@ -88,6 +87,8 @@ struct SwiftType: Equatable, CustomStringConvertible {
             switch nominal.name {
             case .optionalTypeSugarName where nominal.genericParameterTypes.count == 1:
                 repr += "\(nominal.genericParameterTypes[0])?"
+            case .optionalAutounwrappedTypeSugarName where nominal.genericParameterTypes.count == 1:
+                repr += "\(nominal.genericParameterTypes[0])!"
             case .arrayTypeSugarName where nominal.genericParameterTypes.count == 1:
                 repr += "[\(nominal.genericParameterTypes[0])]"
             case .dictionaryTypeSugarName where nominal.genericParameterTypes.count == 2:
@@ -121,14 +122,6 @@ struct SwiftType: Equatable, CustomStringConvertible {
         case .composition(let composition):
             repr += composition.elements.map(\.description).joined(separator: " & ")
         }
-        if isIUO {
-            switch kind {
-            case .tuple, .nominal:
-                repr += "!"
-            case .closure, .composition:
-                repr = "(\(repr))!"
-            }
-        }
         if hasEllipsis {
             repr += "..."
         }
@@ -191,7 +184,11 @@ struct SwiftType: Equatable, CustomStringConvertible {
     }
 
     var isOptional: Bool {
-        isNominal(named: .optional) || isNominal(named: .optionalTypeSugarName)
+        isNominal(named: .optional) || isNominal(named: .optionalTypeSugarName) || isNominal(named: .optionalAutounwrappedTypeSugarName)
+    }
+
+    var isIUO: Bool {
+        isNominal(named: .optionalAutounwrappedTypeSugarName)
     }
 
     var isSelf: Bool {
@@ -286,9 +283,8 @@ struct SwiftType: Equatable, CustomStringConvertible {
                 ret = unwrapped
             }
         }
-        ret.isIUO = true
 
-        return ret.description
+        return ret.iuoWrapped().description
     }
 
     func defaultVal(with overrides: [String: String]? = nil, overrideKey: String = "", isInitParam: Bool = false) -> String? {
@@ -343,7 +339,7 @@ struct SwiftType: Equatable, CustomStringConvertible {
             var processedType = subtype.processTypeParams(with: typeParams)
             processedType.attributes.removeAll(where: { $0 == .inout })
             processedType.attributes.removeAll(where: { $0 == .escaping })
-            processedType.isIUO = false
+            processedType.unwrapIUO()
             return processedType
         }
 
@@ -456,10 +452,9 @@ struct SwiftType: Equatable, CustomStringConvertible {
                 returnAsType = unwrapped
                 asSuffix = "?"
             } else if returnType.isIUO {
-                displayableReturnType = .Any
-                displayableReturnType.isIUO = true
+                displayableReturnType = .Any.iuoWrapped()
                 var asType = returnType
-                asType.isIUO = false
+                asType.unwrapIUO()
                 returnAsType = asType
             } else if returnType.isSelf {
                 returnAsType = .Self
@@ -476,11 +471,18 @@ struct SwiftType: Equatable, CustomStringConvertible {
             returnTypeCast = " as! " + .`Self`
         }
 
+        // `!` (IUO) is illegal inside a closure type, so an IUO param/return must render as a plain
+        // optional here (it still witnesses the IUO requirement).
+        displayableReturnType.unwrapIUO()
         var resultType = SwiftType(
             kind: .closure(.init(
                 isAsync: isAsync,
                 throwing: throwing,
-                arguments: params.map { .init(type: $0.processTypeParams(with: typeParams)) },
+                arguments: params.map {
+                    var argType = $0.processTypeParams(with: typeParams)
+                    argType.unwrapIUO()
+                    return .init(type: argType)
+                },
                 returning: displayableReturnType
             ))
         )
@@ -630,8 +632,7 @@ extension SwiftType {
         case .implicitlyUnwrappedOptionalType(let syntax):
             // T!
             let base = SwiftType(typeSyntax: syntax.wrappedType)
-            self = base.optionalWrapped()
-            self.isIUO = true
+            self = base.iuoWrapped()
 
         case .identifierType(let syntax):
             // T<U>
@@ -720,6 +721,12 @@ extension SwiftType {
         )
     }
 
+    func iuoWrapped() -> SwiftType {
+        return copy(
+            kind: .nominal(.init(name: .optionalAutounwrappedTypeSugarName, genericParameterTypes: [.init(kind: kind)]))
+        )
+    }
+
     func optionalUnwrapped() -> SwiftType? {
         guard isOptional else {
             return nil
@@ -730,6 +737,17 @@ extension SwiftType {
         return nominal.genericParameterTypes.first
     }
 
+    mutating func unwrapIUO() {
+        guard isNominal(named: .optionalAutounwrappedTypeSugarName) else {
+            return
+        }
+        guard case .nominal(let nominal) = kind,
+            let wrapped = nominal.genericParameterTypes.first else {
+            return
+        }
+        self = wrapped
+    }
+
     func copy(kind: Kind) -> Self {
         var copy = self
         copy.kind = kind
diff --git a/Tests/TestFuncs/TestBasicFuncs/FixtureNonSimpleFuncs.swift b/Tests/TestFuncs/TestBasicFuncs/FixtureNonSimpleFuncs.swift
index 31c14c37..b1afa7e9 100644
--- a/Tests/TestFuncs/TestBasicFuncs/FixtureNonSimpleFuncs.swift
+++ b/Tests/TestFuncs/TestBasicFuncs/FixtureNonSimpleFuncs.swift
@@ -35,6 +35,40 @@ import MockoloFramework
     }
 }
 
+@Fixture enum iuoFuncs {
+    /// @mockable
+    public protocol IUOFunc {
+        func f(x: Int!) -> Int!
+        subscript(i: Int!) -> Int! { get }
+    }
+
+    @Fixture enum expected {
+        public class IUOFuncMock: IUOFunc {
+            public init() { }
+            public private(set) var fCallCount = 0
+            public var fHandler: ((Int) -> Int)?
+            public func f(x: Int!) -> Int! {
+                fCallCount += 1
+                if let fHandler = fHandler {
+                    return fHandler(x)
+                }
+                return nil
+            }
+            public private(set) var subscriptCallCount = 0
+            public var subscriptHandler: ((Int) -> Int)?
+            public subscript(i: Int!) -> Int! {
+                get {
+                subscriptCallCount += 1
+                if let subscriptHandler = subscriptHandler {
+                    return subscriptHandler(i)
+                }
+                return nil
+                }
+            }
+        }
+    }
+}
+
 @Fixture enum subscripts {
     /// @mockable
     protocol SubscriptProtocol {
diff --git a/Tests/TestFuncs/TestBasicFuncs/FuncBasicTests.swift b/Tests/TestFuncs/TestBasicFuncs/FuncBasicTests.swift
index ea5f0804..1af22d77 100644
--- a/Tests/TestFuncs/TestBasicFuncs/FuncBasicTests.swift
+++ b/Tests/TestFuncs/TestBasicFuncs/FuncBasicTests.swift
@@ -9,6 +9,11 @@ class BasicFuncTests: MockoloTestCase {
                dstContent: subscripts.expected._source)
     }
 
+    func testIUOFuncs() {
+        verify(srcContent: iuoFuncs._source,
+               dstContent: iuoFuncs.expected._source)
+    }
+
     func testGetOnlySubscripts() {
         verify(srcContent: getOnlySubscripts._source,
                dstContent: getOnlySubscripts.expected._source)
diff --git a/Tests/TestVars/FixtureNonSimpleVars.swift b/Tests/TestVars/FixtureNonSimpleVars.swift
index 05330f31..821bfeb2 100644
--- a/Tests/TestVars/FixtureNonSimpleVars.swift
+++ b/Tests/TestVars/FixtureNonSimpleVars.swift
@@ -47,3 +47,24 @@ import MockoloFramework
         }
     }
 }
+
+@Fixture enum iuoVars {
+    /// @mockable
+    public protocol IUOVar {
+        var a: Int! { get }
+        var b: Int! { get set }
+    }
+
+    @Fixture enum expected {
+        public class IUOVarMock: IUOVar {
+            public init() { }
+            public init(a: Int! = nil, b: Int! = nil) {
+                self.a = a
+                self.b = b
+            }
+            public var a: Int! = nil
+            public private(set) var bSetCallCount = 0
+            public var b: Int! = nil { didSet { bSetCallCount += 1 } }
+        }
+    }
+}
diff --git a/Tests/TestVars/VarTests.swift b/Tests/TestVars/VarTests.swift
index bb510f91..28a6b8bb 100644
--- a/Tests/TestVars/VarTests.swift
+++ b/Tests/TestVars/VarTests.swift
@@ -21,6 +21,11 @@ class VarTests: MockoloTestCase {
                allowSetCallCount: true)
     }
 
+    func testIUOVars() {
+        verify(srcContent: iuoVars._source,
+               dstContent: iuoVars.expected._source)
+    }
+
 #if compiler(>=6.0)
     func testAsyncThrows() {
         verify(srcContent: asyncThrowsVars._source,