From df8a6928d643fcaedcb9473c2e3115c1276d3786 Mon Sep 17 00:00:00 2001
From: Hiroshi Yamauchi <hjyamauchi@gmail.com>
Date: Thu, 11 Dec 2025 13:51:21 -0800
Subject: [PATCH 1/2] Fix the swiftCxxStdlib link with explicit module build on
 Windows

We currently support static linking swiftCxxStdlib only on
Windows. When C++ interop is enabled but swiftCxxStdlib isn't actually
used or only when its underlying std module is used, we incorrectly
emitted the dynamic version of the lib name and caused the link
error. This change fixes it by always adding it to the dependency.

Issue https://github.com/swiftlang/swift/issues/85876
---
 .../ModuleDependencyScanner.cpp               | 18 ++++++++
 .../win-explicit-cxx-interop-cxxstdlib.swift  | 43 +++++++++++++++++++
 2 files changed, 61 insertions(+)
 create mode 100644 test/ScanDependencies/win-explicit-cxx-interop-cxxstdlib.swift

diff --git a/lib/DependencyScan/ModuleDependencyScanner.cpp b/lib/DependencyScan/ModuleDependencyScanner.cpp
index f40137d3e6f75..fffa1683539b7 100644
--- a/lib/DependencyScan/ModuleDependencyScanner.cpp
+++ b/lib/DependencyScan/ModuleDependencyScanner.cpp
@@ -620,6 +620,24 @@ ModuleDependencyScanner::getMainModuleDependencyInfo(ModuleDecl *mainModule) {
       break;
     }
 
+    if (ScanASTContext.LangOpts.EnableCXXInterop) {
+      StringRef mainModuleName = mainModule->getName().str();
+      if (mainModuleName != CXX_MODULE_NAME)
+        mainDependencies.addModuleImport(CXX_MODULE_NAME, /* isExported */ false,
+                                         AccessLevel::Public,
+                                         &alreadyAddedModules);
+      if (llvm::none_of(llvm::ArrayRef<StringRef>{CXX_MODULE_NAME,
+                            ScanASTContext.Id_CxxStdlib.str(), "std"},
+                        [mainModuleName](StringRef Name) {
+                          return mainModuleName == Name;
+                        }))
+        if (ScanASTContext.LangOpts.Target.getOS() == llvm::Triple::Win32)
+          mainDependencies.addModuleImport(ScanASTContext.Id_CxxStdlib.str(),
+                                           /* isExported */ false,
+                                           AccessLevel::Public,
+                                           &alreadyAddedModules);
+    }
+
     // Add any implicit module names.
     for (const auto &import : importInfo.AdditionalUnloadedImports) {
       mainDependencies.addModuleImport(
diff --git a/test/ScanDependencies/win-explicit-cxx-interop-cxxstdlib.swift b/test/ScanDependencies/win-explicit-cxx-interop-cxxstdlib.swift
new file mode 100644
index 0000000000000..c36be1e30a2b8
--- /dev/null
+++ b/test/ScanDependencies/win-explicit-cxx-interop-cxxstdlib.swift
@@ -0,0 +1,43 @@
+// REQUIRES: OS=windows-msvc
+
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+// RUN: %target-swift-frontend -scan-dependencies -module-name NoCpp \
+// RUN:   -module-cache-path %t/clang-module-cache -disable-objc-interop \
+// RUN:   -cxx-interoperability-mode=default \
+// RUN:   %t/NoCpp.swift -o %t/NoCpp-deps.json -I %t 2>&1
+// RUN: %target-swift-frontend -scan-dependencies -module-name Cpp \
+// RUN:   -module-cache-path %t/clang-module-cache -disable-objc-interop \
+// RUN:   -cxx-interoperability-mode=default \
+// RUN:   %t/Cpp.swift -o %t/Cpp-deps.json -I %t 2>&1
+// RUN: %target-swift-frontend -scan-dependencies -module-name Crt \
+// RUN:   -module-cache-path %t/clang-module-cache -disable-objc-interop \
+// RUN:   -cxx-interoperability-mode=default \
+// RUN:   %t/Crt.swift -o %t/Crt-deps.json -I %t 2>&1
+// RUN: %FileCheck %s --check-prefix=NOCPP < %t/NoCpp-deps.json
+// RUN: %FileCheck %s --check-prefix=CPP < %t/Cpp-deps.json
+// RUN: %FileCheck %s --check-prefix=CRT < %t/Crt-deps.json
+
+// NOCPP:      "mainModuleName": "NoCpp"
+// NOCPP:      "linkName": "swiftCxxStdlib",
+// NOCPP-NEXT: "isStatic": true,
+
+// CPP:        "mainModuleName": "Cpp"
+// CPP:        "linkName": "swiftCxxStdlib",
+// CPP-NEXT:   "isStatic": true,
+
+// CRT:        "mainModuleName": "Crt"
+// CRT:        "linkName": "swiftCxxStdlib",
+// CRT-NEXT:   "isStatic": true,
+
+//--- NoCpp.swift
+// Empty
+
+//--- Cpp.swift
+import CxxStdlib
+
+//--- Crt.swift
+import CRT
+
+
+

From 854d3db83ce71a083837d65846f5d27d3e8a3462 Mon Sep 17 00:00:00 2001
From: Hiroshi Yamauchi <hjyamauchi@gmail.com>
Date: Fri, 12 Dec 2025 18:19:56 -0800
Subject: [PATCH 2/2] Test

---
 lib/DependencyScan/ModuleDependencyScanner.cpp | 1 -
 1 file changed, 1 deletion(-)

diff --git a/lib/DependencyScan/ModuleDependencyScanner.cpp b/lib/DependencyScan/ModuleDependencyScanner.cpp
index fffa1683539b7..a4e34b9254661 100644
--- a/lib/DependencyScan/ModuleDependencyScanner.cpp
+++ b/lib/DependencyScan/ModuleDependencyScanner.cpp
@@ -631,7 +631,6 @@ ModuleDependencyScanner::getMainModuleDependencyInfo(ModuleDecl *mainModule) {
                         [mainModuleName](StringRef Name) {
                           return mainModuleName == Name;
                         }))
-        if (ScanASTContext.LangOpts.Target.getOS() == llvm::Triple::Win32)
           mainDependencies.addModuleImport(ScanASTContext.Id_CxxStdlib.str(),
                                            /* isExported */ false,
                                            AccessLevel::Public,