Potential linker issue with optional parameters.

[plotfi]

@drodriguez some time ago noticed some issues around C++-Interop and linking with the use of default parameters in Swift. After some investigation on #66501, which looks like something similar to what we encountered, I tried to setup a similar case described below. Now, this actually repros with or without C++-Interop, but I dont understand why.

Description

Module Map:

module CXX {
  header "CXX.h"
}

Code:

///---------------------------------------------------------------------------------------------------------------///
/// CXX.h
/// ObjC++ Module:

#include <Foundation/Foundation.h>

typedef NS_OPTIONS(NSUInteger, FooBarCenteringOptions) {
  FooBarCenteringNone NS_SWIFT_NAME(None) = 0,
  FooBarCenteringX NS_SWIFT_NAME(x) = 1,
  FooBarCenteringY NS_SWIFT_NAME(y) = 2,
  FooBarCenteringXY NS_SWIFT_NAME(xy) = 3
} NS_SWIFT_NAME(FooBarComponent.CenteringOptions);

typedef NS_OPTIONS(NSUInteger, FooBarSizingOptions) {
  FooBarSizingOptionDefault,
  FooBarSizingOptionMinimumX = 1,
  FooBarSizingOptionMinimumY = 2,
  FooBarSizingOptionMinimumXY = 3
} NS_SWIFT_NAME(FooBarComponent.SizingOptions);


@interface FooBarLayoutComponent : NSObject
@end

NS_SWIFT_NAME(FooBarComponent)
@interface FooBarComponent : FooBarLayoutComponent
@end
///---------------------------------------------------------------------------------------------------------------///

///---------------------------------------------------------------------------------------------------------------///
/// CxxSwiftModule.swift
/// Swift Module, Uses C++:


import Foundation
import CXX

public
func center(centeringOptions: FooBarComponent.CenteringOptions = .xy,
            sizingOptions: FooBarComponent.SizingOptions = [])
  -> FooBarComponent.SizingOptions {
  sizingOptions
}

///---------------------------------------------------------------------------------------------------------------///

///---------------------------------------------------------------------------------------------------------------///
/// CxxSwiftModuleUser.swift
/// Swift User of C++ using Swift Module:

import Foundation
import CXX
import CxxSwiftModule

public func f() -> FooBarComponent.SizingOptions {
  center()
}

@main
class C {
  static func main() {
    let c = center()
    if c == FooBarComponent.SizingOptions.minimumXY {
      print("foo")
    } else {
      print("bar")
    }
  }
}
///---------------------------------------------------------------------------------------------------------------///

Steps to reproduce

/Users/plotfi/opt/dev/S/build/RelWithDebInfo/BinaryCache/toolchain/bin/swift-frontend \
  -frontend -c \
  -sdk  /Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk \
  -enable-objc-interop \
  -cxx-interoperability-mode=default \ ### <- This line is optional
  -disable-autolinking-runtime-compatibility-concurrency \
  -disable-autolinking-runtime-compatibility-dynamic-replacements \
  -disable-autolinking-runtime-compatibility \
  -o CxxSwiftModule.o  \
  CxxSwiftModule.swift -I./ -emit-module -parse-as-library


/Users/plotfi/opt/dev/S/build/RelWithDebInfo/BinaryCache/toolchain/bin/swift-frontend \
  -frontend -c \
  -sdk  /Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk \
  -enable-objc-interop \
  -disable-autolinking-runtime-compatibility-concurrency \
  -disable-autolinking-runtime-compatibility-dynamic-replacements \
  -disable-autolinking-runtime-compatibility \
  -o CxxSwiftModuleUser.o  \
  CxxSwiftModuleUser.swift -disable-cxx-interop-requirement-at-import -I./ -parse-as-library


 /Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ld \
  -syslibroot /Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk \
-o a.out \
-L/Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/macosx \
-L/usr/lib/swift \
CxxSwiftModuleUser.o CxxSwiftModuleUser.o

Expected behavior

Linker spits out:

Undefined symbols for architecture arm64:
  "_$s14CxxSwiftModule6center16centeringOptions06sizingF0So012FooBarSizingF0VSo0hi9CenteringF0V_AFtF", referenced from:
      _$s18CxxSwiftModuleUser1fSo19FooBarSizingOptionsVyF in CxxSwiftModuleUser.o
      _$s18CxxSwiftModuleUser1CC4mainyyFZ in CxxSwiftModuleUser.o
      _$s18CxxSwiftModuleUser1fSo19FooBarSizingOptionsVyF in CxxSwiftModuleUser.o
      _$s18CxxSwiftModuleUser1CC4mainyyFZ in CxxSwiftModuleUser.o
     (maybe you meant: _$s14CxxSwiftModule6center16centeringOptions06sizingF0So012FooBarSizingF0VSo0hi9CenteringF0V_AFtFfA0_, _$s14CxxSwiftModule6center16centeringOptions06sizingF0So012FooBarSizingF0VSo0hi9CenteringF0V_AFtFfA_ )
ld: symbol(s) not found for architecture arm64

Demangled:

ld: symbol(s) not found for architecture arm64Undefined symbols for architecture arm64:
  "CxxSwiftModule.center(centeringOptions: __C.FooBarCenteringOptions, sizingOptions: __C.FooBarSizingOptions) -> __C.FooBarSizingOptions", referenced from:
      CxxSwiftModuleUser.f() -> __C.FooBarSizingOptions in CxxSwiftModuleUser.o
      static CxxSwiftModuleUser.C.main() -> () in CxxSwiftModuleUser.o
      CxxSwiftModuleUser.f() -> __C.FooBarSizingOptions in CxxSwiftModuleUser.o
      static CxxSwiftModuleUser.C.main() -> () in CxxSwiftModuleUser.o
     (maybe you meant: default argument 1 of CxxSwiftModule.center(centeringOptions: __C.FooBarCenteringOptions, sizingOptions: __C.FooBarSizingOptions) -> __C.FooBarSizingOptions, default argument 0 of CxxSwiftModule.center(centeringOptions: __C.FooBarCenteringOptions, sizingOptions: __C.FooBarSizingOptions) -> __C.FooBarSizingOptions )

Now I am not sure why this is the case here.

[plotfi]

One thing to note, this is work-aroundable. Just drop the default params. But still want to understand if the linker is right here and why.