[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

Author: Gabor Horvath

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;
 

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']