From 33e41f1281c0328fd2174f79fcd709eb42079a3e Mon Sep 17 00:00:00 2001
From: Gabor Horvath <gaborh@apple.com>
Date: Wed, 18 Jun 2025 11:40:07 +0100
Subject: [PATCH] [cxx-interop] Fix duplicate symbol error with default
 arguments

We synthesize a Swift function that only calls a C++ funtion that
produces the default argument. We produce this function for each module
that imports the C++ function with the default argument. This symbol
needs to be public as it is created in the context of the Clang module
and used from the Swift module. To avoid the linker errors, we always
emit this function into the client which is also the right thing to do
as the whole body is a single function call.

rdar://138513915
---
 lib/ClangImporter/SwiftDeclSynthesizer.cpp    |  3 ++
 .../default-arguments-multifile.swift         | 46 +++++++++++++++++++
 test/Interop/Cxx/function/lit.local.cfg       |  6 +++
 3 files changed, 55 insertions(+)
 create mode 100644 test/Interop/Cxx/function/default-arguments-multifile.swift
 create mode 100644 test/Interop/Cxx/function/lit.local.cfg

diff --git a/lib/ClangImporter/SwiftDeclSynthesizer.cpp b/lib/ClangImporter/SwiftDeclSynthesizer.cpp
index 24e1a8d94e565..9ce81fbc0c0a8 100644
--- a/lib/ClangImporter/SwiftDeclSynthesizer.cpp
+++ b/lib/ClangImporter/SwiftDeclSynthesizer.cpp
@@ -12,6 +12,7 @@
 
 #include "SwiftDeclSynthesizer.h"
 #include "swift/AST/ASTMangler.h"
+#include "swift/AST/Attr.h"
 #include "swift/AST/AttrKind.h"
 #include "swift/AST/Builtins.h"
 #include "swift/AST/Decl.h"
@@ -2508,6 +2509,8 @@ SwiftDeclSynthesizer::makeDefaultArgument(const clang::ParmVarDecl *param,
       ImporterImpl.ImportedHeaderUnit);
   funcDecl->setBodySynthesizer(synthesizeDefaultArgumentBody, (void *)param);
   funcDecl->setAccess(AccessLevel::Public);
+  funcDecl->getAttrs().add(new (ctx)
+                               AlwaysEmitIntoClientAttr(/*IsImplicit=*/true));
 
   ImporterImpl.defaultArgGenerators[param] = funcDecl;
 
diff --git a/test/Interop/Cxx/function/default-arguments-multifile.swift b/test/Interop/Cxx/function/default-arguments-multifile.swift
new file mode 100644
index 0000000000000..2dd3bc315d7b0
--- /dev/null
+++ b/test/Interop/Cxx/function/default-arguments-multifile.swift
@@ -0,0 +1,46 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+// RUN: mkdir -p %t/artifacts
+
+// Multiple usages in the same module.
+// RUN: %target-build-swift %t/main.swift %t/b.swift %t/c.swift -cxx-interoperability-mode=upcoming-swift -I %S/Inputs -o %t/artifacts/out
+// RUN: %empty-directory(%t/artifacts)
+
+// Multiple usages across different modules.
+// RUN: %target-build-swift -emit-library -module-name BarLibrary -emit-module -emit-module-path %t/artifacts/BarLibrary.swiftmodule %t/b.swift %t/c.swift -cxx-interoperability-mode=upcoming-swift -I %S/Inputs -o %t/artifacts/%target-library-name(BarLibrary)
+// RUN: %target-build-swift %t/uses-library.swift -cxx-interoperability-mode=upcoming-swift -I %S/Inputs -I %t/artifacts -L %t/artifacts -lBarLibrary -o %t/artifacts/uses-library
+
+// FIXME: Windows test can be enabled once merge-modules step is removed from the old driver,
+// or the Windows CI starts to use the new driver to compile the compiler.
+// The windows toolchains still use the new driver so the CI failure worked
+// around here should not affect users on Windows.
+// UNSUPPORTED: OS=windows-msvc
+
+//--- main.swift
+import DefaultArguments
+public func foo() {
+  let _ = isZero()
+}
+foo()
+bar()
+baz()
+
+//--- b.swift
+import DefaultArguments
+public func bar() {
+  let _ = isZero()
+}
+
+//--- c.swift
+import DefaultArguments
+public func baz() {
+  let _ = isZero(123)
+}
+
+//--- uses-library.swift
+import DefaultArguments
+import BarLibrary
+
+let _ = isZero()
+bar()
+baz()
diff --git a/test/Interop/Cxx/function/lit.local.cfg b/test/Interop/Cxx/function/lit.local.cfg
new file mode 100644
index 0000000000000..4e6b2cfd49774
--- /dev/null
+++ b/test/Interop/Cxx/function/lit.local.cfg
@@ -0,0 +1,6 @@
+# Make a local copy of the environment.
+config.environment = dict(config.environment)
+
+# FIXME: deserialization failure with the old driver in default-arguments-multifile.swift
+if 'SWIFT_USE_OLD_DRIVER' in config.environment: del config.environment['SWIFT_USE_OLD_DRIVER']
+if 'SWIFT_AVOID_WARNING_USING_OLD_DRIVER' in config.environment: del config.environment['SWIFT_AVOID_WARNING_USING_OLD_DRIVER']