diff --git a/include/swift/AST/SourceFile.h b/include/swift/AST/SourceFile.h
index f247013cfc6b6..e40b4e52a8b53 100644
--- a/include/swift/AST/SourceFile.h
+++ b/include/swift/AST/SourceFile.h
@@ -144,6 +144,9 @@ class SourceFile final : public FileUnit {
   /// This is \c None until it is filled in by the import resolution phase.
   llvm::Optional<ArrayRef<AttributedImport<ImportedModule>>> Imports;
 
+  /// The underlying clang module, if imported in this file.
+  ModuleDecl *ImportedUnderlyingModule = nullptr;
+
   /// Which imports have made use of @preconcurrency.
   llvm::SmallDenseSet<AttributedImport<ImportedModule>>
       PreconcurrencyImportsUsed;
@@ -693,6 +696,17 @@ class SourceFile final : public FileUnit {
     return isScriptMode() || hasMainDecl();
   }
 
+  ModuleDecl *getUnderlyingModuleIfOverlay() const override {
+    return ImportedUnderlyingModule;
+  }
+
+  const clang::Module *getUnderlyingClangModule() const override {
+    if (!ImportedUnderlyingModule)
+      return nullptr;
+
+    return ImportedUnderlyingModule->findUnderlyingClangModule();
+  }
+
   /// Get the root refinement context for the file. The root context may be
   /// null if the context hierarchy has not been built yet. Use
   /// TypeChecker::getOrBuildTypeRefinementContext() to get a built
diff --git a/lib/AST/Module.cpp b/lib/AST/Module.cpp
index 4da2e42b75abe..30386b3c08382 100644
--- a/lib/AST/Module.cpp
+++ b/lib/AST/Module.cpp
@@ -2566,6 +2566,20 @@ void
 SourceFile::setImports(ArrayRef<AttributedImport<ImportedModule>> imports) {
   assert(!Imports && "Already computed imports");
   Imports = getASTContext().AllocateCopy(imports);
+
+  // Find and cache the import of the underlying module, if present.
+  auto parentModuleName = getParentModule()->getName();
+  for (auto import : imports) {
+    if (!import.options.contains(ImportFlags::Exported))
+      continue;
+
+    auto importedModule = import.module.importedModule;
+    if (importedModule->getName() == parentModuleName &&
+        importedModule->findUnderlyingClangModule()) {
+      ImportedUnderlyingModule = import.module.importedModule;
+      break;
+    }
+  }
 }
 
 bool SourceFile::hasImportUsedPreconcurrency(
diff --git a/lib/AST/ModuleDependencies.cpp b/lib/AST/ModuleDependencies.cpp
index f383654fa6e69..50717699b9683 100644
--- a/lib/AST/ModuleDependencies.cpp
+++ b/lib/AST/ModuleDependencies.cpp
@@ -438,8 +438,6 @@ swift::dependencies::checkImportNotTautological(const ImportPath::Module moduleP
                        filename, modulePath.front().Item);
 
   return false;
-
-  return false;
 }
 
 void SwiftDependencyTracker::addCommonSearchPathDeps(
diff --git a/test/Inputs/clang-importer-sdk/swift-modules/ObjectiveC.swift b/test/Inputs/clang-importer-sdk/swift-modules/ObjectiveC.swift
index f6071839a99a9..278fb9d340760 100644
--- a/test/Inputs/clang-importer-sdk/swift-modules/ObjectiveC.swift
+++ b/test/Inputs/clang-importer-sdk/swift-modules/ObjectiveC.swift
@@ -82,7 +82,7 @@ public func ~=(x: NSObject, y: NSObject) -> Bool {
   return true
 }
 
-extension NSObject : @retroactive Equatable, @retroactive Hashable {
+extension NSObject : Equatable, Hashable {
   public static func == (lhs: NSObject, rhs: NSObject) -> Bool {
     return lhs.isEqual(rhs)
   }
diff --git a/test/Sema/extension_retroactive_conformances_underlying_clang_module.swift b/test/Sema/extension_retroactive_conformances_underlying_clang_module.swift
new file mode 100644
index 0000000000000..64c946c00940e
--- /dev/null
+++ b/test/Sema/extension_retroactive_conformances_underlying_clang_module.swift
@@ -0,0 +1,49 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+// RUN: %target-swift-frontend -swift-version 5 %t/OtherLibrary.swift -emit-module -module-name OtherLibrary -o %t
+// RUN: %target-swift-frontend -typecheck %t/Library.swift -module-name Library -verify -swift-version 5 -import-underlying-module -I %t
+// RUN: %target-swift-frontend -typecheck %t/Library.swift -module-name Library -verify -swift-version 5 -DEXPLICIT_IMPORT -I %t
+
+// REQUIRES: objc_interop
+
+//--- module.modulemap
+
+module Library {
+  header "Library.h"
+}
+
+//--- Library.h
+
+@import Foundation;
+
+@interface UnderlyingLibraryClass : NSObject
+@end
+
+@protocol UnderlyingLibraryProtocol
+@end
+
+//--- OtherLibrary.swift
+
+public class OtherLibraryClass {}
+public protocol OtherLibraryProtocol {}
+
+//--- Library.swift
+
+#if EXPLICIT_IMPORT
+@_exported import Library
+#endif
+import OtherLibrary
+
+public class LibraryClass {}
+public protocol LibraryProtocol {}
+
+extension LibraryClass: UnderlyingLibraryProtocol {}
+extension LibraryClass: LibraryProtocol {}
+extension LibraryClass: OtherLibraryProtocol {}
+extension UnderlyingLibraryClass: OtherLibraryProtocol {}
+extension UnderlyingLibraryClass: LibraryProtocol {}
+extension UnderlyingLibraryClass: UnderlyingLibraryProtocol {}
+extension OtherLibraryClass: UnderlyingLibraryProtocol {}
+extension OtherLibraryClass: LibraryProtocol {}
+extension OtherLibraryClass: OtherLibraryProtocol {} // expected-warning {{extension declares a conformance of imported type 'OtherLibraryClass' to imported protocol 'OtherLibraryProtocol'}}
+// expected-note @-1 {{add '@retroactive' to silence this warning}} {{30-50=@retroactive OtherLibraryProtocol}}