[6.2][cxx-interop] Fix duplicate symbol error with default arguments

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"
@@ -2522,6 +2523,8 @@ 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;
 

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()

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']