When one module references another module, the autogenerated `-Swift.h` header breaks

[mrousavy]

Description

I have two Swift modules/pods: FirstPod and SecondPod. Both use C++ interop, and both call Swift classes/functions from C++.

FirstPod has this class:

open class First { … }

SecondPod has this class:

import FirstPod
public class Second: First { … }

Because Second inherits from First, the SecondPod-Swift.h header file produces build errors since it cannot find Second's base class (First) as that is nowhere included:

- Unknown class name 'First'

Code:

class SWIFT_SYMBOL("s:13Second06FirstC") Second : public FirstPod::First {
public:
  using First::First;
  using First::operator=;
  static SWIFT_INLINE_THUNK Second init() SWIFT_SYMBOL("s:13Second06FirstCACycfc");
protected:
  SWIFT_INLINE_THUNK Second(void * _Nonnull ptr) noexcept : First(ptr) {}
private:
  friend class _impl::_impl_Second;
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wc++17-extensions"
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wreserved-identifier"
  typedef char $s13Second06FirstCD;
  static inline constexpr $s13Second06FirstCD __swift_mangled_name = 0;
#pragma clang diagnostic pop
#pragma clang diagnostic pop
};

• Forward-declaring doesn't help either, since it needs a concrete type for a base class.
• Including FirstPod-Swift.h before I include SecondPod-Swift.h doesn't work either because (as far as I know) you cannot include the -Swift.h header of another module inside your module.

I'm out of ideas at this point. Any suggestions?

Reproduction

public class First { … }

import FirstPod
public class Second: First { … }

…then try calling Second from within a C++ file in SecondPod

Expected behavior

I expect there to be some sort of forward-declaration mechanism, or a void* boxing mechanism to ensure foreign types can be handled.

Environment

swift-driver version: 1.120.5 Apple Swift version 6.1.2 (swiftlang-6.1.2.1.2 clang-1700.0.13.5)
Target: arm64-apple-macosx15.0

Additional information

No response

[Xazax-hun]

Including FirstPod-Swift.h before I include SecondPod-Swift.h doesn't work either because (as far as I know) you cannot include the -Swift.h header of another module inside your module.

Out of curiosity, did you try this one? I.e., generating a header that includes both of the generated headers in the right order and using that from C++.

[mrousavy]

I did try this in a CocoaPods Xcode project, and the -Swift.h header from one module is just not available in another module.
And if I manually add it (via it's fully qualified path in ~/DerivedData/…), I'm getting duplicate symbols build errors.