diff --git a/lib/ClangImporter/ClangImporter.cpp b/lib/ClangImporter/ClangImporter.cpp
index 6182b43e83a50..655cd59b263fd 100644
--- a/lib/ClangImporter/ClangImporter.cpp
+++ b/lib/ClangImporter/ClangImporter.cpp
@@ -6273,7 +6273,7 @@ static ValueDecl *cloneBaseMemberDecl(ValueDecl *decl, DeclContext *newContext,
     auto out = new (rawMemory) TypeAliasDecl(
         typeDecl->getLoc(), typeDecl->getLoc(), typeDecl->getName(),
         typeDecl->getLoc(), nullptr, newContext);
-    out->setUnderlyingType(typeDecl->getInterfaceType());
+    out->setUnderlyingType(typeDecl->getDeclaredInterfaceType());
     out->setAccess(access);
     inheritance.setUnavailableIfNecessary(decl, out);
     return out;
diff --git a/test/Interop/Cxx/class/inheritance/Inputs/module.modulemap b/test/Interop/Cxx/class/inheritance/Inputs/module.modulemap
index c4702d40aa4fb..007e8f550b970 100644
--- a/test/Interop/Cxx/class/inheritance/Inputs/module.modulemap
+++ b/test/Interop/Cxx/class/inheritance/Inputs/module.modulemap
@@ -16,8 +16,8 @@ module ReferenceToDerived {
   requires cplusplus
 }
 
-module SubTypes {
-  header "sub-types.h"
+module NestedTypes {
+  header "nested-types.h"
 }
 
 module Subscripts {
diff --git a/test/Interop/Cxx/class/inheritance/Inputs/sub-types.h b/test/Interop/Cxx/class/inheritance/Inputs/nested-types.h
similarity index 83%
rename from test/Interop/Cxx/class/inheritance/Inputs/sub-types.h
rename to test/Interop/Cxx/class/inheritance/Inputs/nested-types.h
index b5f013fe2eb32..866c21925a909 100644
--- a/test/Interop/Cxx/class/inheritance/Inputs/sub-types.h
+++ b/test/Interop/Cxx/class/inheritance/Inputs/nested-types.h
@@ -20,3 +20,5 @@ struct Base {
 };
 
 struct Derived : Base {};
+struct Derived1 : Base {};
+struct Derived2 : Derived1 {};
diff --git a/test/Interop/Cxx/class/inheritance/sub-types-module-interface.swift b/test/Interop/Cxx/class/inheritance/nested-types-module-interface.swift
similarity index 55%
rename from test/Interop/Cxx/class/inheritance/sub-types-module-interface.swift
rename to test/Interop/Cxx/class/inheritance/nested-types-module-interface.swift
index 4029fc7ccb08c..201556b074e3e 100644
--- a/test/Interop/Cxx/class/inheritance/sub-types-module-interface.swift
+++ b/test/Interop/Cxx/class/inheritance/nested-types-module-interface.swift
@@ -1,4 +1,4 @@
-// RUN: %target-swift-ide-test -print-module -module-to-print=SubTypes -I %S/Inputs -source-filename=x -enable-experimental-cxx-interop | %FileCheck %s
+// RUN: %target-swift-ide-test -print-module -module-to-print=NestedTypes -I %S/Inputs -source-filename=x -cxx-interoperability-mode=default | %FileCheck %s
 
 // CHECK:      struct Base {
 // CHECK-NEXT:   init()
@@ -39,11 +39,29 @@
 // CHECK-NEXT:   }
 // CHECK-NEXT: }
 
-// CHECK-NEXT: struct Derived {
+// CHECK:      struct Derived {
 // CHECK-NEXT:   init()
-// CHECK-NEXT:   typealias EnumClass = Base.EnumClass.Type
-// CHECK-NEXT:   typealias Enum = Base.Enum.Type
-// CHECK-NEXT:   typealias Struct = Base.Struct.Type
-// CHECK-NEXT:   typealias Parent = Base.Parent.Type
-// CHECK-NEXT:   typealias Union = Base.Union.Type
+// CHECK-NEXT:   typealias EnumClass = Base.EnumClass
+// CHECK-NEXT:   typealias Enum = Base.Enum
+// CHECK-NEXT:   typealias Struct = Base.Struct
+// CHECK-NEXT:   typealias Parent = Base.Parent
+// CHECK-NEXT:   typealias Union = Base.Union
+// CHECK-NEXT: }
+
+// CHECK:      struct Derived1 {
+// CHECK-NEXT:   init()
+// CHECK-NEXT:   typealias EnumClass = Base.EnumClass
+// CHECK-NEXT:   typealias Enum = Base.Enum
+// CHECK-NEXT:   typealias Struct = Base.Struct
+// CHECK-NEXT:   typealias Parent = Base.Parent
+// CHECK-NEXT:   typealias Union = Base.Union
+// CHECK-NEXT: }
+
+// CHECK:      struct Derived2 {
+// CHECK-NEXT:   init()
+// CHECK-NEXT:   typealias EnumClass = Base.EnumClass
+// CHECK-NEXT:   typealias Enum = Base.Enum
+// CHECK-NEXT:   typealias Struct = Base.Struct
+// CHECK-NEXT:   typealias Parent = Base.Parent
+// CHECK-NEXT:   typealias Union = Base.Union
 // CHECK-NEXT: }
diff --git a/test/Interop/Cxx/class/inheritance/nested-types-typechecker.swift b/test/Interop/Cxx/class/inheritance/nested-types-typechecker.swift
new file mode 100644
index 0000000000000..f56f8672d275c
--- /dev/null
+++ b/test/Interop/Cxx/class/inheritance/nested-types-typechecker.swift
@@ -0,0 +1,113 @@
+// RUN: %target-typecheck-verify-swift -I %S/Inputs -cxx-interoperability-mode=default
+
+// This test ensures that nested type inherited from the base type are
+// interchangeable with the base types themselves.
+
+import NestedTypes
+
+func parent(
+  bc: Base.EnumClass,
+  be: Base.Enum,
+  bs: Base.Struct,
+  bp: Base.Parent,
+  bpc: Base.Parent.Child,
+  bu: Base.Union,
+) {
+  let dc: Derived.EnumClass = bc
+  let de: Derived.Enum = be
+  let ds: Derived.Struct = bs
+  let dp: Derived.Parent = bp
+  let dpc: Derived.Parent.Child = bpc
+  let du: Derived.Union = bu
+
+  let _: Base.EnumClass = dc
+  let _: Base.Enum = de
+  let _: Base.Struct = ds
+  let _: Base.Parent = dp
+  let _: Base.Parent.Child = dpc
+  let _: Base.Union = du
+}
+
+func grandparent(
+  bc: Base.EnumClass,
+  be: Base.Enum,
+  bs: Base.Struct,
+  bp: Base.Parent,
+  bpc: Base.Parent.Child,
+  bu: Base.Union,
+) {
+  let dc: Derived2.EnumClass = bc
+  let de: Derived2.Enum = be
+  let ds: Derived2.Struct = bs
+  let dp: Derived2.Parent = bp
+  let dpc: Derived2.Parent.Child = bpc
+  let du: Derived2.Union = bu
+
+  let _: Derived.EnumClass = dc
+  let _: Derived.Enum = de
+  let _: Derived.Struct = ds
+  let _: Derived.Parent = dp
+  let _: Derived.Parent.Child = dpc
+  let _: Derived.Union = du
+}
+
+func siblings(
+  dc: Derived.EnumClass,
+  de: Derived.Enum,
+  ds: Derived.Struct,
+  dp: Derived.Parent,
+  dpc: Derived.Parent.Child,
+  du: Derived.Union,
+) {
+  let _: Derived1.EnumClass = dc
+  let _: Derived1.Enum = de
+  let _: Derived1.Struct = ds
+  let _: Derived1.Parent = dp
+  let _: Derived1.Parent.Child = dpc
+  let _: Derived1.Union = du
+}
+
+// Instances created from derived class can be type-annotated with base class
+// and vice versa
+func assigner() {
+  let _: Base.EnumClass = Derived.EnumClass.ecb
+  let _: Base.Enum = Derived.ea // expected-error {{type 'Derived' has no member 'ea'}}
+                                // ^FIXME: nested enums are broken, so inherited nested enums are broken too
+  let _: Base.Struct = Derived.Struct(sa: 4, sb: 2)
+  let _: Base.Parent = Derived.Parent()
+  let _: Base.Parent.Child = Derived.Parent.Child(pca: 42)
+  let _: Base.Union = Derived.Union(ua: 42)
+
+  let _: Derived.EnumClass = Base.EnumClass.ecc
+  let _: Derived.Enum = Base.ea // expected-error {{type 'Base' has no member 'ea'}}
+                                // ^FIXME: nested enums are broken, so inherited nested enums are broken too
+  let _: Derived.Struct = Base.Struct(sa: 4, sb: 2)
+  let _: Derived.Parent = Base.Parent()
+  let _: Derived.Parent.Child = Base.Parent.Child(pca: 42)
+  let _: Derived.Union = Base.Union(ua: 42)
+}
+
+// Extensions on base type should be "seen" in derived types too, and vice versa
+extension Base.Parent {
+  static func getChild1() -> Child {
+    return Child(pca: 111)
+  }
+}
+
+extension Derived {
+  func haveChild1() {
+    let _: Parent.Child = Parent.getChild1()
+  }
+}
+
+extension Derived.Parent {
+  static func getChild2() -> Child {
+    return Child(pca: 111)
+  }
+}
+
+extension Base {
+  func haveChild2() {
+    let _: Parent.Child = Parent.getChild2()
+  }
+}
diff --git a/test/Interop/Cxx/class/inheritance/type-aliases-module-interface.swift b/test/Interop/Cxx/class/inheritance/type-aliases-module-interface.swift
index 4dde31f117869..66a28d63424ef 100644
--- a/test/Interop/Cxx/class/inheritance/type-aliases-module-interface.swift
+++ b/test/Interop/Cxx/class/inheritance/type-aliases-module-interface.swift
@@ -1,4 +1,4 @@
-// RUN: %target-swift-ide-test -print-module -module-to-print=TypeAliases -I %S/Inputs -source-filename=x -enable-experimental-cxx-interop | %FileCheck %s
+// RUN: %target-swift-ide-test -print-module -module-to-print=TypeAliases -I %S/Inputs -source-filename=x -cxx-interoperability-mode=default | %FileCheck %s
 
 // CHECK:      struct Base {
 // CHECK-NEXT:   init()
@@ -11,7 +11,7 @@
 
 // CHECK-NEXT: struct Derived {
 // CHECK-NEXT:   init()
-// CHECK-NEXT:   typealias Struct = Base.Struct.Type
+// CHECK-NEXT:   typealias Struct = Base.Struct
 // CHECK-NEXT:   typealias T = Int32
 // CHECK-NEXT:   typealias U = Base.Struct
 // CHECK-NEXT: }
diff --git a/test/Interop/Cxx/class/inheritance/type-aliases-typechecker.swift b/test/Interop/Cxx/class/inheritance/type-aliases-typechecker.swift
new file mode 100644
index 0000000000000..a59bc3e3cfedb
--- /dev/null
+++ b/test/Interop/Cxx/class/inheritance/type-aliases-typechecker.swift
@@ -0,0 +1,51 @@
+// RUN: %target-typecheck-verify-swift -I %S/Inputs -cxx-interoperability-mode=default
+
+// This test ensures that nested type aliases inherited from the base type are 
+// interchangeable with the base types themselves.
+
+import TypeAliases
+
+extension Derived {
+  func takesBaseTypes(bs: Base.Struct, bt: Base.T, bu: Base.U) {
+    let _: Struct = bs
+    let _: U = bs
+
+    let _: T = bt
+    let _: Int32 = bt
+    
+    let _: U = bu
+    let _: Struct = bu
+  }
+
+  func takesDerivedTypes(ds: Struct, dt: T, du: U) {
+    let _: Base.Struct = ds
+    let _: Base.U = ds
+
+    let _: Base.T = dt
+    let _: Int32 = dt
+    
+    let _: Base.U = du
+    let _: Base.Struct = du
+  }
+}
+func takesBaseTypes(bs: Base.Struct, bt: Base.T, bu: Base.U) {
+  let _: Derived.Struct = bs
+  let _: Derived.U = bs
+
+  let _: Derived.T = bt
+  let _: Int32 = bt
+  
+  let _: Derived.U = bu
+  let _: Derived.Struct = bu
+}
+
+func takesDerivedTypes(ds: Derived.Struct, dt: Derived.T, du: Derived.U) {
+  let _: Base.Struct = ds
+  let _: Base.U = ds
+
+  let _: Base.T = dt
+  let _: Int32 = dt
+  
+  let _: Base.U = du
+  let _: Base.Struct = du
+}