diff --git a/include/swift/AST/DeclAttr.def b/include/swift/AST/DeclAttr.def
index 5d7ec7df6eab..cfff48354d0b 100644
--- a/include/swift/AST/DeclAttr.def
+++ b/include/swift/AST/DeclAttr.def
@@ -474,7 +474,7 @@ SIMPLE_DECL_ATTR(_alwaysEmitIntoClient, AlwaysEmitIntoClient,
   83)
 
 SIMPLE_DECL_ATTR(_implementationOnly, ImplementationOnly,
-  OnImport | OnFunc | OnConstructor | OnVar | OnSubscript | OnStruct | OnClass | OnEnum,
+  OnImport | OnFunc | OnConstructor | OnVar | OnSubscript | OnNominalType,
   UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   84)
 
diff --git a/lib/Sema/TypeCheckAccess.cpp b/lib/Sema/TypeCheckAccess.cpp
index 2250437ae022..0d49728effb2 100644
--- a/lib/Sema/TypeCheckAccess.cpp
+++ b/lib/Sema/TypeCheckAccess.cpp
@@ -2163,13 +2163,13 @@ swift::getDisallowedOriginKind(const Decl *decl,
           Feature::AssumeResilientCxxTypes))
     return DisallowedOriginKind::FragileCxxAPI;
 
-  if (isa<StructDecl>(decl) || isa<EnumDecl>(decl)) {
-    if (decl->getASTContext().LangOpts.hasFeature(
-            Feature::CheckImplementationOnly) &&
-        decl->getAttrs().hasAttribute<ImplementationOnlyAttr>()) {
-      return DisallowedOriginKind::ImplementationOnlyMemoryLayout;
-    }
-  }
+  // Implementation-only memory layouts for non-library-evolution mode.
+  // Should we allow references to classes and protocols in non-embedded mode?
+  if (isa<NominalTypeDecl>(decl) &&
+      decl->getASTContext().LangOpts.hasFeature(
+          Feature::CheckImplementationOnly) &&
+      decl->getAttrs().hasAttribute<ImplementationOnlyAttr>())
+    return DisallowedOriginKind::ImplementationOnlyMemoryLayout;
 
   // Report non-public import last as it can be ignored by the caller.
   // See \c diagnoseValueDeclRefExportability.
diff --git a/test/Sema/hidden-memory-layout.swift b/test/Sema/hidden-memory-layout.swift
index 1e6e53b2c5b1..195491aa5fe1 100644
--- a/test/Sema/hidden-memory-layout.swift
+++ b/test/Sema/hidden-memory-layout.swift
@@ -72,6 +72,36 @@ private struct ExposedLayoutPrivate {
   init() { fatalError() } // expected-note {{initializer 'init()' is not '@usableFromInline' or public}}
 }
 
+public class ExposedClassPublic {
+  public init() { fatalError() }
+}
+
+internal class ExposedClassInternal {
+// expected-note @-1 {{type declared here}}
+}
+
+private class ExposedClassPrivate {
+// expected-note @-1 2 {{class 'ExposedClassPrivate' is not '@usableFromInline' or public}}
+// expected-note @-2 2 {{type declared here}}
+  init() { fatalError() } // expected-note {{initializer 'init()' is not '@usableFromInline' or public}}
+}
+
+#if UseImplementationOnly
+@_implementationOnly
+private class HiddenClass {
+// expected-opt-in-note @-1 2 {{class 'HiddenClass' is not '@usableFromInline' or public}}
+// expected-opt-in-note @-2 1 {{initializer 'init()' is not '@usableFromInline' or public}}
+// expected-opt-in-note @-3 6 {{class declared here}}
+// expected-opt-in-note @-4 2 {{type declared here}}
+}
+#else
+private class HiddenClass {
+// expected-not-opt-in-note @-1 2 {{class 'HiddenClass' is not '@usableFromInline' or public}}
+// expected-not-opt-in-note @-2 1 {{initializer 'init()' is not '@usableFromInline' or public}}
+// expected-not-opt-in-note @-3 2 {{type declared here}}
+}
+#endif
+
 #if UseImplementationOnly
 @_implementationOnly
 private struct HiddenLayout {
@@ -118,6 +148,33 @@ private enum HiddenEnum {
 }
 #endif
 
+public protocol ExposedProtocolPublic {
+}
+
+internal protocol ExposedProtocolInternal {
+// expected-note @-1 {{protocol 'ExposedProtocolInternal' is not '@usableFromInline' or public}}
+// expected-note @-2 {{type declared here}}
+}
+
+private protocol ExposedProtocolPrivate {
+// expected-note @-1 {{protocol 'ExposedProtocolPrivate' is not '@usableFromInline' or public}}
+// expected-note @-2 2 {{type declared here}}
+}
+
+#if UseImplementationOnly
+@_implementationOnly
+private protocol HiddenProtocol {
+// expected-opt-in-note @-1 {{protocol 'HiddenProtocol' is not '@usableFromInline' or public}}
+// expected-opt-in-note @-2 9 {{protocol declared here}}
+// expected-opt-in-note @-3 2 {{type declared here}}
+}
+#else
+private protocol HiddenProtocol {
+// expected-not-opt-in-note @-1 1 {{protocol 'HiddenProtocol' is not '@usableFromInline' or public}}
+// expected-not-opt-in-note @-2 2 {{type declared here}}
+}
+#endif
+
 /// Function use sites
 
 @inlinable
@@ -131,6 +188,15 @@ public func explicitlyInlinable() {
   // expected-error @-1 2 {{struct 'HiddenLayout' is private and cannot be referenced from an '@inlinable' function}}
   // expected-error @-2 {{initializer 'init()' is private and cannot be referenced from an '@inlinable' function}}
 
+  let _: ExposedClassPublic = ExposedClassPublic()
+  let _: ExposedClassPrivate = ExposedClassPrivate()
+  // expected-error @-1 2 {{class 'ExposedClassPrivate' is private and cannot be referenced from an '@inlinable' function}}
+  // expected-error @-2 {{initializer 'init()' is private and cannot be referenced from an '@inlinable' function}}
+
+  let _: HiddenClass = HiddenClass()
+  // expected-error @-1 2 {{class 'HiddenClass' is private and cannot be referenced from an '@inlinable' function}}
+  // expected-error @-2 {{initializer 'init()' is private and cannot be referenced from an '@inlinable' function}}
+
   let _: ExposedEnumPublic = ExposedEnumPublic.A
   let _: ExposedEnumPrivate = ExposedEnumPrivate.A
   // expected-error @-1 2 {{enum 'ExposedEnumPrivate' is private and cannot be referenced from an '@inlinable' function}}
@@ -138,6 +204,14 @@ public func explicitlyInlinable() {
   let _: HiddenEnum = HiddenEnum.A
   // expected-error @-1 2 {{enum 'HiddenEnum' is private and cannot be referenced from an '@inlinable' function}}
   // expected-error @-2 {{enum case 'A' is private and cannot be referenced from an '@inlinable' function}}
+
+  let _: ExposedProtocolPublic
+  let _: ExposedProtocolInternal
+  // expected-error @-1 {{protocol 'ExposedProtocolInternal' is internal and cannot be referenced from an '@inlinable' function}}
+  let _: ExposedProtocolPrivate
+  // expected-error @-1 {{protocol 'ExposedProtocolPrivate' is private and cannot be referenced from an '@inlinable' function}}
+  let _: HiddenProtocol
+  // expected-error @-1 {{protocol 'HiddenProtocol' is private and cannot be referenced from an '@inlinable' function}}
 }
 
 public func implicitlyInlinablePublic() {
@@ -146,10 +220,21 @@ public func implicitlyInlinablePublic() {
   let _: HiddenLayout = HiddenLayout()
   // expected-embedded-opt-in-error @-1 2 {{struct 'HiddenLayout' cannot be used in an embedded function not marked '@_neverEmitIntoClient' because 'HiddenLayout' is marked '@_implementationOnly'}}
 
+  let _: ExposedClassPublic = ExposedClassPublic()
+  let _: ExposedClassPrivate = ExposedClassPrivate()
+  let _: HiddenClass = HiddenClass()
+  // expected-embedded-opt-in-error @-1 2 {{class 'HiddenClass' cannot be used in an embedded function not marked '@_neverEmitIntoClient' because 'HiddenClass' is marked '@_implementationOnly'}}
+
   let _: ExposedEnumPublic = ExposedEnumPublic.A
   let _: ExposedEnumPrivate = ExposedEnumPrivate.A
   let _: HiddenEnum = HiddenEnum.A
   // expected-embedded-opt-in-error @-1 2 {{enum 'HiddenEnum' cannot be used in an embedded function not marked '@_neverEmitIntoClient' because 'HiddenEnum' is marked '@_implementationOnly'}}
+
+  let _: ExposedProtocolPublic
+  let _: ExposedProtocolInternal
+  let _: ExposedProtocolPrivate
+  let _: HiddenProtocol
+  // expected-embedded-opt-in-error @-1 {{protocol 'HiddenProtocol' cannot be used in an embedded function not marked '@_neverEmitIntoClient' because 'HiddenProtocol' is marked '@_implementationOnly'}}
 }
 
 private func implicitlyInlinablePrivate() {
@@ -158,10 +243,21 @@ private func implicitlyInlinablePrivate() {
   let _: HiddenLayout = HiddenLayout()
   // expected-embedded-opt-in-error @-1 2 {{struct 'HiddenLayout' cannot be used in an embedded function not marked '@_neverEmitIntoClient' because 'HiddenLayout' is marked '@_implementationOnly'}}
 
+  let _: ExposedClassPublic = ExposedClassPublic()
+  let _: ExposedClassPrivate = ExposedClassPrivate()
+  let _: HiddenClass = HiddenClass()
+  // expected-embedded-opt-in-error @-1 2 {{class 'HiddenClass' cannot be used in an embedded function not marked '@_neverEmitIntoClient' because 'HiddenClass' is marked '@_implementationOnly'}}
+
   let _: ExposedEnumPublic = ExposedEnumPublic.A
   let _: ExposedEnumPrivate = ExposedEnumPrivate.A
   let _: HiddenEnum = HiddenEnum.A
   // expected-embedded-opt-in-error @-1 2 {{enum 'HiddenEnum' cannot be used in an embedded function not marked '@_neverEmitIntoClient' because 'HiddenEnum' is marked '@_implementationOnly'}}
+
+  let _: ExposedProtocolPublic
+  let _: ExposedProtocolInternal
+  let _: ExposedProtocolPrivate
+  let _: HiddenProtocol
+  // expected-embedded-opt-in-error @-1 {{protocol 'HiddenProtocol' cannot be used in an embedded function not marked '@_neverEmitIntoClient' because 'HiddenProtocol' is marked '@_implementationOnly'}}
 }
 
 @_neverEmitIntoClient
@@ -169,9 +265,19 @@ public func explicitNonInliable() {
   let _: ExposedLayoutPublic = ExposedLayoutPublic()
   let _: ExposedLayoutPrivate = ExposedLayoutPrivate()
   let _: HiddenLayout = HiddenLayout()
+
+  let _: ExposedClassPublic = ExposedClassPublic()
+  let _: ExposedClassPrivate = ExposedClassPrivate()
+  let _: HiddenClass = HiddenClass()
+
   let _: ExposedEnumPublic = ExposedEnumPublic.A
   let _: ExposedEnumPrivate = ExposedEnumPrivate.A
   let _: HiddenEnum = HiddenEnum.A
+
+  let _: ExposedProtocolPublic
+  let _: ExposedProtocolInternal
+  let _: ExposedProtocolPrivate
+  let _: HiddenProtocol
 }
 
 @_neverEmitIntoClient
@@ -179,9 +285,19 @@ internal func explicitNonInliableInternal() {
   let _: ExposedLayoutPublic = ExposedLayoutPublic()
   let _: ExposedLayoutPrivate = ExposedLayoutPrivate()
   let _: HiddenLayout = HiddenLayout()
+
+  let _: ExposedClassPublic = ExposedClassPublic()
+  let _: ExposedClassPrivate = ExposedClassPrivate()
+  let _: HiddenClass = HiddenClass()
+
   let _: ExposedEnumPublic = ExposedEnumPublic.A
   let _: ExposedEnumPrivate = ExposedEnumPrivate.A
   let _: HiddenEnum = HiddenEnum.A
+
+  let _: ExposedProtocolPublic
+  let _: ExposedProtocolInternal
+  let _: ExposedProtocolPrivate
+  let _: HiddenProtocol
 }
 
 /// Struct use sites
@@ -198,17 +314,33 @@ public struct ExposedLayoutPublicUser: ProtocolFromDirect {
   private var a: ExposedLayoutPublic
   private var aa: ExposedLayoutInternal
   private var b: ExposedLayoutPrivate
-
   private var c: HiddenLayout
   // expected-opt-in-error @-1 {{cannot use struct 'HiddenLayout' in a property declaration marked public or in a '@frozen' or '@usableFromInline' context; 'HiddenLayout' is marked '@_implementationOnly'}}
 
+  private var ca: ExposedClassPublic
+  private var cb: ExposedClassInternal
+  private var cc: ExposedClassPrivate
+  private var cd: HiddenClass
+  // expected-opt-in-error @-1 {{cannot use class 'HiddenClass' in a property declaration marked public or in a '@frozen' or '@usableFromInline' context; 'HiddenClass' is marked '@_implementationOnly'}}
+
   private var d: ExposedEnumPublic
   private var e: ExposedEnumPrivate
   private var f: HiddenEnum
   // expected-opt-in-error @-1 {{cannot use enum 'HiddenEnum' in a property declaration marked public or in a '@frozen' or '@usableFromInline' context; 'HiddenEnum' is marked '@_implementationOnly'}}
 
+  private var pp: ProtocolFromDirect
+  // expected-opt-in-error @-1 {{cannot use protocol 'ProtocolFromDirect' in a property declaration marked public or in a '@frozen' or '@usableFromInline' context; 'directs' has been imported as implementation-only}}
+
+  private var g: ExposedProtocolPublic
+  private var h: ExposedProtocolInternal
+  private var i: ExposedProtocolPrivate
+  private var j: HiddenProtocol
+  // expected-opt-in-error @-1 {{cannot use protocol 'HiddenProtocol' in a property declaration marked public or in a '@frozen' or '@usableFromInline' context; 'HiddenProtocol' is marked '@_implementationOnly'}}
+
   private func privateFunc(h: HiddenLayout) {}
   // expected-embedded-opt-in-error @-1 {{struct 'HiddenLayout' cannot be used in an embedded function not marked '@_neverEmitIntoClient' because 'HiddenLayout' is marked '@_implementationOnly'}}
+  private func privateFuncClass(h: HiddenClass) {}
+  // expected-embedded-opt-in-error @-1 {{class 'HiddenClass' cannot be used in an embedded function not marked '@_neverEmitIntoClient' because 'HiddenClass' is marked '@_implementationOnly'}}
 }
 
 internal struct ExposedLayoutInternalUser: ProtocolFromDirect {
@@ -223,13 +355,27 @@ internal struct ExposedLayoutInternalUser: ProtocolFromDirect {
   private var c: HiddenLayout
   // expected-opt-in-error @-1 {{cannot use struct 'HiddenLayout' in a property declaration marked public or in a '@frozen' or '@usableFromInline' context; 'HiddenLayout' is marked '@_implementationOnly'}}
 
+  private var ca: ExposedClassPublic
+  private var cb: ExposedClassInternal
+  private var cc: ExposedClassPrivate
+  private var cd: HiddenClass
+  // expected-opt-in-error @-1 {{cannot use class 'HiddenClass' in a property declaration marked public or in a '@frozen' or '@usableFromInline' context; 'HiddenClass' is marked '@_implementationOnly'}}
+
   private var d: ExposedEnumPublic
   private var e: ExposedEnumPrivate
   private var f: HiddenEnum
   // expected-opt-in-error @-1 {{cannot use enum 'HiddenEnum' in a property declaration marked public or in a '@frozen' or '@usableFromInline' context; 'HiddenEnum' is marked '@_implementationOnly'}}
 
+  private var g: ExposedProtocolPublic
+  private var h: ExposedProtocolInternal
+  private var i: ExposedProtocolPrivate
+  private var j: HiddenProtocol
+  // expected-opt-in-error @-1 {{cannot use protocol 'HiddenProtocol' in a property declaration marked public or in a '@frozen' or '@usableFromInline' context; 'HiddenProtocol' is marked '@_implementationOnly'}}
+
   private func privateFunc(h: HiddenLayout) {}
   // expected-embedded-opt-in-error @-1 {{struct 'HiddenLayout' cannot be used in an embedded function not marked '@_neverEmitIntoClient' because 'HiddenLayout' is marked '@_implementationOnly'}}
+  private func privateFuncClass(h: HiddenClass) {}
+  // expected-embedded-opt-in-error @-1 {{class 'HiddenClass' cannot be used in an embedded function not marked '@_neverEmitIntoClient' because 'HiddenClass' is marked '@_implementationOnly'}}
 }
 
 private struct ExposedLayoutPrivateUser: ProtocolFromDirect {
@@ -244,13 +390,27 @@ private struct ExposedLayoutPrivateUser: ProtocolFromDirect {
   private var c: HiddenLayout
   // expected-opt-in-error @-1 {{cannot use struct 'HiddenLayout' in a property declaration marked public or in a '@frozen' or '@usableFromInline' context; 'HiddenLayout' is marked '@_implementationOnly'}}
 
+  private var ca: ExposedClassPublic
+  private var cb: ExposedClassInternal
+  private var cc: ExposedClassPrivate
+  private var cd: HiddenClass
+  // expected-opt-in-error @-1 {{cannot use class 'HiddenClass' in a property declaration marked public or in a '@frozen' or '@usableFromInline' context; 'HiddenClass' is marked '@_implementationOnly'}}
+
   private var d: ExposedEnumPublic
   private var e: ExposedEnumPrivate
   private var f: HiddenEnum
   // expected-opt-in-error @-1 {{cannot use enum 'HiddenEnum' in a property declaration marked public or in a '@frozen' or '@usableFromInline' context; 'HiddenEnum' is marked '@_implementationOnly'}}
 
+  private var g: ExposedProtocolPublic
+  private var h: ExposedProtocolInternal
+  private var i: ExposedProtocolPrivate
+  private var j: HiddenProtocol
+  // expected-opt-in-error @-1 {{cannot use protocol 'HiddenProtocol' in a property declaration marked public or in a '@frozen' or '@usableFromInline' context; 'HiddenProtocol' is marked '@_implementationOnly'}}
+
   private func privateFunc(h: HiddenLayout) {}
   // expected-embedded-opt-in-error @-1 {{struct 'HiddenLayout' cannot be used in an embedded function not marked '@_neverEmitIntoClient' because 'HiddenLayout' is marked '@_implementationOnly'}}
+  private func privateFuncClass(h: HiddenClass) {}
+  // expected-embedded-opt-in-error @-1 {{class 'HiddenClass' cannot be used in an embedded function not marked '@_neverEmitIntoClient' because 'HiddenClass' is marked '@_implementationOnly'}}
 }
 
 #if UseImplementationOnly
@@ -261,19 +421,32 @@ private struct HiddenLayoutUser {
   private var a: ExposedLayoutPublic
   private var aa: ExposedLayoutInternal
   private var b: ExposedLayoutPrivate
+
+  private var ca: ExposedClassPublic
+  private var cb: ExposedClassInternal
+  private var cc: ExposedClassPrivate
+
   private var c: HiddenLayout
+  private var cd: HiddenClass
+
   private var d: ExposedEnumPublic
   private var e: ExposedEnumPrivate
   private var f: HiddenEnum
 
+  private var g: ExposedProtocolPublic
+  private var h: ExposedProtocolInternal
+  private var i: ExposedProtocolPrivate
+  private var j: HiddenProtocol
+
   @_neverEmitIntoClient
   private func privateFunc(h: HiddenLayout) {}
+  @_neverEmitIntoClient
+  private func privateFuncClass(h: HiddenClass) {}
 }
 
 @_implementationOnly // expected-opt-in-error {{'@_implementationOnly' may not be used on public declarations}}
 public struct PublicHiddenStruct {}
 
-
 #endif
 
 /// Enums use sites
@@ -281,30 +454,67 @@ public struct PublicHiddenStruct {}
 public enum PublicEnumUser: ProtocolFromDirect {
 // expected-error @-1 {{cannot use protocol 'ProtocolFromDirect' in a public or '@usableFromInline' conformance; 'directs' has been imported as implementation-only}}
     case a(StructFromDirect) // expected-error {{cannot use struct 'StructFromDirect' here; 'directs' has been imported as implementation-only}}
-    case b(HiddenLayout) // expected-error {{enum case in a public enum uses a private type}}
-    // expected-opt-in-error @-1 {{cannot use struct 'HiddenLayout' here; 'HiddenLayout' is marked '@_implementationOnly'}}
+
+    case e(ExposedLayoutPublic)
     case c(ExposedLayoutInternal) // expected-error {{enum case in a public enum uses an internal type}}
     case d(ExposedLayoutPrivate) // expected-error {{enum case in a public enum uses a private type}}
-    case e(ExposedLayoutPublic)
+    case b(HiddenLayout) // expected-error {{enum case in a public enum uses a private type}}
+    // expected-opt-in-error @-1 {{cannot use struct 'HiddenLayout' here; 'HiddenLayout' is marked '@_implementationOnly'}}
+
+    case ce(ExposedClassPublic)
+    case cc(ExposedClassInternal) // expected-error {{enum case in a public enum uses an internal type}}
+    case cd(ExposedClassPrivate) // expected-error {{enum case in a public enum uses a private type}}
+    case cb(HiddenClass) // expected-error {{enum case in a public enum uses a private type}}
+    // expected-opt-in-error @-1 {{cannot use class 'HiddenClass' here; 'HiddenClass' is marked '@_implementationOnly'}}
+
+    case f(ExposedProtocolPublic)
+    case g(ExposedProtocolInternal) // expected-error {{enum case in a public enum uses an internal type}}
+    case h(ExposedProtocolPrivate) // expected-error {{enum case in a public enum uses a private type}}
+    case i(HiddenProtocol) // expected-opt-in-error {{cannot use protocol 'HiddenProtocol' here; 'HiddenProtocol' is marked '@_implementationOnly'}}
+    // expected-error @-1 {{enum case in a public enum uses a private type}}
 }
 
 internal enum InternalEnumUser: ProtocolFromDirect {
 // expected-opt-in-error @-1 {{cannot use protocol 'ProtocolFromDirect' in a public or '@usableFromInline' conformance; 'directs' has been imported as implementation-only}}
     case a(StructFromDirect) // expected-opt-in-error {{cannot use struct 'StructFromDirect' here; 'directs' has been imported as implementation-only}}
-    case b(HiddenLayout) // expected-opt-in-error {{cannot use struct 'HiddenLayout' here; 'HiddenLayout' is marked '@_implementationOnly'}}
-    // expected-error @-1 {{enum case in an internal enum uses a private type}}
+
+    case e(ExposedLayoutPublic)
     case c(ExposedLayoutInternal)
     case d(ExposedLayoutPrivate) // expected-error {{enum case in an internal enum uses a private type}}
-    case e(ExposedLayoutPublic)
+    case b(HiddenLayout) // expected-opt-in-error {{cannot use struct 'HiddenLayout' here; 'HiddenLayout' is marked '@_implementationOnly'}}
+    // expected-error @-1 {{enum case in an internal enum uses a private type}}
+
+    case ce(ExposedClassPublic)
+    case cc(ExposedClassInternal)
+    case cd(ExposedClassPrivate) // expected-error {{enum case in an internal enum uses a private type}}
+    case cb(HiddenClass) // expected-opt-in-error {{cannot use class 'HiddenClass' here; 'HiddenClass' is marked '@_implementationOnly'}}
+    // expected-error @-1 {{enum case in an internal enum uses a private type}}
+
+    case f(ExposedProtocolPublic)
+    case g(ExposedProtocolInternal)
+    case h(ExposedProtocolPrivate) // expected-error {{enum case in an internal enum uses a private type}}
+    case i(HiddenProtocol) // expected-opt-in-error {{cannot use protocol 'HiddenProtocol' here; 'HiddenProtocol' is marked '@_implementationOnly'}}
+    // expected-error @-1 {{enum case in an internal enum uses a private type}}
 }
 
 private enum PrivateEnumUser: ProtocolFromDirect {
 // expected-opt-in-error @-1 {{cannot use protocol 'ProtocolFromDirect' in a public or '@usableFromInline' conformance; 'directs' has been imported as implementation-only}}
     case a(StructFromDirect) // expected-opt-in-error {{cannot use struct 'StructFromDirect' here; 'directs' has been imported as implementation-only}}
-    case b(HiddenLayout) // expected-opt-in-error {{cannot use struct 'HiddenLayout' here; 'HiddenLayout' is marked '@_implementationOnly'}}
+
+    case e(ExposedLayoutPublic)
     case c(ExposedLayoutInternal)
     case d(ExposedLayoutPrivate)
-    case e(ExposedLayoutPublic)
+    case b(HiddenLayout) // expected-opt-in-error {{cannot use struct 'HiddenLayout' here; 'HiddenLayout' is marked '@_implementationOnly'}}
+
+    case ce(ExposedClassPublic)
+    case cc(ExposedClassInternal)
+    case cd(ExposedClassPrivate)
+    case cb(HiddenClass) // expected-opt-in-error {{cannot use class 'HiddenClass' here; 'HiddenClass' is marked '@_implementationOnly'}}
+
+    case f(ExposedProtocolPublic)
+    case g(ExposedProtocolInternal)
+    case h(ExposedProtocolPrivate)
+    case i(HiddenProtocol) // expected-opt-in-error {{cannot use protocol 'HiddenProtocol' here; 'HiddenProtocol' is marked '@_implementationOnly'}}
 }
 
 internal enum InternalEnumWithRawType : RawTypeFromDirect { // expected-opt-in-error {{cannot use struct 'RawTypeFromDirect' in a public or '@usableFromInline' conformance; 'directs' has been imported as implementation-only}}
@@ -316,10 +526,21 @@ internal enum InternalEnumWithRawType : RawTypeFromDirect { // expected-opt-in-e
 @_implementationOnly
 private enum PrivateHiddenEnumUser: ProtocolFromDirect {
     case a(StructFromDirect)
-    case b(HiddenLayout)
+
+    case e(ExposedLayoutPublic)
     case c(ExposedLayoutInternal)
     case d(ExposedLayoutPrivate)
-    case e(ExposedLayoutPublic)
+    case b(HiddenLayout)
+
+    case ce(ExposedClassPublic)
+    case cc(ExposedClassInternal)
+    case cd(ExposedClassPrivate)
+    case cb(HiddenClass)
+
+    case f(ExposedProtocolPublic)
+    case g(ExposedProtocolInternal)
+    case h(ExposedProtocolPrivate)
+    case i(HiddenProtocol)
 }
 
 @_implementationOnly // expected-opt-in-error {{'@_implementationOnly' may not be used on public declarations}}
@@ -335,7 +556,7 @@ internal enum InternalEnumWithRawTypeIO : RawTypeFromDirect {
 
 /// Classes use sites
 
-public class PublicClass: ProtocolFromDirect {
+public class PublicClassUser: ProtocolFromDirect {
 // expected-error @-1 {{cannot use protocol 'ProtocolFromDirect' in a public or '@usableFromInline' conformance; 'directs' has been imported as implementation-only}}
 
   public init() { fatalError() }
@@ -356,11 +577,17 @@ public class PublicClass: ProtocolFromDirect {
   private var f: HiddenEnum
   // expected-opt-in-error @-1 {{cannot use enum 'HiddenEnum' in a property declaration marked public or in a '@frozen' or '@usableFromInline' context; 'HiddenEnum' is marked '@_implementationOnly'}}
 
+  private var g: ExposedProtocolPublic
+  private var h: ExposedProtocolInternal
+  private var i: ExposedProtocolPrivate
+  private var j: HiddenProtocol
+  // expected-opt-in-error @-1 {{cannot use protocol 'HiddenProtocol' in a property declaration marked public or in a '@frozen' or '@usableFromInline' context; 'HiddenProtocol' is marked '@_implementationOnly'}}
+
   @_neverEmitIntoClient
   private func privateFunc(h: HiddenLayout) {}
 }
 
-internal class InternalClass: ProtocolFromDirect {
+internal class InternalClassUser: ProtocolFromDirect {
 // expected-opt-in-error @-1 {{cannot use protocol 'ProtocolFromDirect' in a public or '@usableFromInline' conformance; 'directs' has been imported as implementation-only}}
 
   public init() { fatalError() }
@@ -381,10 +608,16 @@ internal class InternalClass: ProtocolFromDirect {
   private var f: HiddenEnum
   // expected-opt-in-error @-1 {{cannot use enum 'HiddenEnum' in a property declaration marked public or in a '@frozen' or '@usableFromInline' context; 'HiddenEnum' is marked '@_implementationOnly'}}
 
+  private var g: ExposedProtocolPublic
+  private var h: ExposedProtocolInternal
+  private var i: ExposedProtocolPrivate
+  private var j: HiddenProtocol
+  // expected-opt-in-error @-1 {{cannot use protocol 'HiddenProtocol' in a property declaration marked public or in a '@frozen' or '@usableFromInline' context; 'HiddenProtocol' is marked '@_implementationOnly'}}
+
   private func privateFunc(h: HiddenLayout) {} // expected-embedded-opt-in-error {{struct 'HiddenLayout' cannot be used in an embedded function not marked '@_neverEmitIntoClient' because 'HiddenLayout' is marked '@_implementationOnly'}}
 }
 
-private class PrivateClass: ProtocolFromDirect {
+private class PrivateClassUser: ProtocolFromDirect {
 // expected-opt-in-error @-1 {{cannot use protocol 'ProtocolFromDirect' in a public or '@usableFromInline' conformance; 'directs' has been imported as implementation-only}}
 
   public init() { fatalError() }
@@ -405,12 +638,18 @@ private class PrivateClass: ProtocolFromDirect {
   private var f: HiddenEnum
   // expected-opt-in-error @-1 {{cannot use enum 'HiddenEnum' in a property declaration marked public or in a '@frozen' or '@usableFromInline' context; 'HiddenEnum' is marked '@_implementationOnly'}}
 
+  private var g: ExposedProtocolPublic
+  private var h: ExposedProtocolInternal
+  private var i: ExposedProtocolPrivate
+  private var j: HiddenProtocol
+  // expected-opt-in-error @-1 {{cannot use protocol 'HiddenProtocol' in a property declaration marked public or in a '@frozen' or '@usableFromInline' context; 'HiddenProtocol' is marked '@_implementationOnly'}}
+
   private func privateFunc(h: HiddenLayout) {} // expected-embedded-opt-in-error {{struct 'HiddenLayout' cannot be used in an embedded function not marked '@_neverEmitIntoClient' because 'HiddenLayout' is marked '@_implementationOnly'}}
 }
 
 #if UseImplementationOnly
 @_implementationOnly
-internal class HiddenClass: ProtocolFromDirect {
+internal class HiddenClassUser: ProtocolFromDirect {
   public init() { fatalError() }
 
   public var publicField: StructFromDirect
@@ -421,11 +660,41 @@ internal class HiddenClass: ProtocolFromDirect {
   private var b: ExposedLayoutPrivate
   private var c: HiddenLayout
 
+  private var ca: ExposedClassPublic
+  private var cb: ExposedClassInternal
+  private var cc: ExposedClassPrivate
+  private var cd: HiddenClass
+
   private var d: ExposedEnumPublic
   private var e: ExposedEnumPrivate
   private var f: HiddenEnum
+
+  private var g: ExposedProtocolPublic
+  private var h: ExposedProtocolInternal
+  private var i: ExposedProtocolPrivate
+  private var j: HiddenProtocol
 }
 
 @_implementationOnly // expected-opt-in-error {{'@_implementationOnly' may not be used on public declarations}}
 public enum PublicHiddenClass {}
 #endif
+
+/// Protocol use sites
+
+public protocol PublicProtocol : ProtocolFromDirect {
+// expected-error @-1 {{cannot use protocol 'ProtocolFromDirect' here; 'directs' has been imported as implementation-only}}
+}
+
+internal protocol InternalProtocol : ProtocolFromDirect {
+// expected-opt-in-error @-1 {{cannot use protocol 'ProtocolFromDirect' here; 'directs' has been imported as implementation-only}}
+}
+
+private protocol PrivateProtocol : ProtocolFromDirect {
+// expected-opt-in-error @-1 {{cannot use protocol 'ProtocolFromDirect' here; 'directs' has been imported as implementation-only}}
+}
+
+#if UseImplementationOnly
+@_implementationOnly
+internal protocol PrivateProtocolHidden : ProtocolFromDirect {
+}
+#endif