diff --git a/lib/DependencyScan/ModuleDependencyScanner.cpp b/lib/DependencyScan/ModuleDependencyScanner.cpp
index 5f773c58eaca6..f926119c19141 100644
--- a/lib/DependencyScan/ModuleDependencyScanner.cpp
+++ b/lib/DependencyScan/ModuleDependencyScanner.cpp
@@ -1541,41 +1541,41 @@ void ModuleDependencyScanner::resolveSwiftOverlayDependenciesForModule(
     recordResult(clangDep);
 
   // C++ Interop requires additional handling
-  if (ScanCompilerInvocation.getLangOptions().EnableCXXInterop &&
-      moduleID.Kind == ModuleDependencyKind::SwiftInterface) {
+  bool lookupCxxStdLibOverlay = ScanCompilerInvocation.getLangOptions().EnableCXXInterop;
+  if (lookupCxxStdLibOverlay && moduleID.Kind == ModuleDependencyKind::SwiftInterface) {
     const auto &moduleInfo = cache.findKnownDependency(moduleID);
     const auto commandLine = moduleInfo.getCommandline();
-
     // If the textual interface was built without C++ interop, do not query
     // the C++ Standard Library Swift overlay for its compilation.
     //
-    // FIXME: We always declare the 'Darwin' module as formally having been
-    // built without C++Interop, for compatibility with prior versions. Once we
-    // are certain that we are only building against modules built with support
-    // of
-    // '-formal-cxx-interoperability-mode', this hard-coded check should be
-    // removed.
-    if (moduleID.ModuleName != "Darwin" &&
-        llvm::find(commandLine, "-formal-cxx-interoperability-mode=off") ==
-            commandLine.end()) {
-      for (const auto &clangDepName : allClangDependencies) {
-        // If this Clang module is a part of the C++ stdlib, and we haven't
-        // loaded the overlay for it so far, it is a split libc++ module (e.g.
-        // std_vector). Load the CxxStdlib overlay explicitly.
-        const auto &clangDepInfo =
-            cache.findDependency(clangDepName, ModuleDependencyKind::Clang)
-                .value()
-                ->getAsClangModule();
-        if (importer::isCxxStdModule(clangDepName, clangDepInfo->IsSystem) &&
-            !swiftOverlayDependencies.contains(
-                {clangDepName, ModuleDependencyKind::SwiftInterface}) &&
-            !swiftOverlayDependencies.contains(
-                {clangDepName, ModuleDependencyKind::SwiftBinary})) {
-          scanForSwiftDependency(
-              getModuleImportIdentifier(ScanASTContext.Id_CxxStdlib.str()));
-          recordResult(ScanASTContext.Id_CxxStdlib.str().str());
-          break;
-        }
+    // FIXME: We always declare the 'Darwin' module as formally having been built
+    // without C++Interop, for compatibility with prior versions. Once we are certain
+    // that we are only building against modules built with support of
+    // '-formal-cxx-interoperability-mode', this hard-coded check should be removed.
+    if (moduleID.ModuleName == "Darwin" ||
+        llvm::find(commandLine, "-formal-cxx-interoperability-mode=off") !=
+         commandLine.end())
+      lookupCxxStdLibOverlay = false;
+  }
+
+  if (lookupCxxStdLibOverlay) {
+    for (const auto &clangDepName : allClangDependencies) {
+      // If this Clang module is a part of the C++ stdlib, and we haven't
+      // loaded the overlay for it so far, it is a split libc++ module (e.g.
+      // std_vector). Load the CxxStdlib overlay explicitly.
+      const auto &clangDepInfo =
+          cache.findDependency(clangDepName, ModuleDependencyKind::Clang)
+              .value()
+              ->getAsClangModule();
+      if (importer::isCxxStdModule(clangDepName, clangDepInfo->IsSystem) &&
+          !swiftOverlayDependencies.contains(
+              {clangDepName, ModuleDependencyKind::SwiftInterface}) &&
+          !swiftOverlayDependencies.contains(
+              {clangDepName, ModuleDependencyKind::SwiftBinary})) {
+        scanForSwiftDependency(
+            getModuleImportIdentifier(ScanASTContext.Id_CxxStdlib.str()));
+        recordResult(ScanASTContext.Id_CxxStdlib.str().str());
+        break;
       }
     }
   }
diff --git a/test/ScanDependencies/cxx-overlay-source-lookup.swift b/test/ScanDependencies/cxx-overlay-source-lookup.swift
new file mode 100644
index 0000000000000..bcc978d3931e3
--- /dev/null
+++ b/test/ScanDependencies/cxx-overlay-source-lookup.swift
@@ -0,0 +1,41 @@
+// RUN: %empty-directory(%t)
+// RUN: %empty-directory(%t/deps)
+// RUN: split-file %s %t
+
+// RUN: %target-swift-frontend -scan-dependencies -o %t/deps.json %t/client.swift -I %t/deps -cxx-interoperability-mode=default -disable-implicit-string-processing-module-import -disable-implicit-concurrency-module-import
+// RUN: cat %t/deps.json | %FileCheck %s
+
+/// --------Main module
+// CHECK-LABEL: "modulePath": "deps.swiftmodule",
+// CHECK-NEXT: "sourceFiles": [
+// CHECK-NEXT: cxx-overlay-source-lookup.swift
+// CHECK-NEXT: ],
+// CHECK: "directDependencies": [
+// CHECK-DAG: "swift": "Swift"
+// CHECK-DAG: "swift": "SwiftOnoneSupport"
+// CHECK-DAG: "swift": "Cxx"
+// CHECK-DAG: "swift": "CxxStdlib"
+// CHECK-DAG: "clang": "CxxShim"
+// CHECK-DAG: "clang": "Foo"
+// CHECK: ],
+
+//--- deps/bar.h
+void bar(void);
+
+//--- deps/foo.h
+#include "bar.h"
+void foo(void);
+
+//--- deps/module.modulemap
+module std_Bar [system] {
+  header "bar.h"
+  export *
+}
+
+module Foo {
+  header "foo.h"
+  export *
+}
+
+//--- client.swift
+import Foo