On Windows in C++ interop mode, the generated C++ header includes internal types

[hwjeremy]

Description

When building a target in C++ interop mode on Windows, and generating a Clang header, we expect that only public members will be exposed in the header. It appears that in some circumstances, internal members will also be exposed, causing any compilation including the header to fail.

Reproduction

Consider a target being built for using in a C++ project:

        .target(
            name: "Foo",
            swiftSettings: [
                .interoperabilityMode(.Cxx),
                .unsafeFlags(["-emit-clang-header-path", "Foo.h"])
            ]
        ),

With a type...

public struct Bar {

    let baz: Int
    
    public func qux() -> Void { } 
    
 }

And elsewhere...

extension Bar {

    internal struct Fred {
        let thud: Int
    }

}

On macOS, as expected, our C++ header is generated without any declaration of or reference to Fred, per the C++ interoperability documentation stating that only public symbols are exposed via the C++ header.

On Windows, we see Bar and its method qux are declared as expected. We also see a reference to Fred, as...

  using Fred=Fred;

Visible in the wider context of various Bar declaration, in the public members section:

class SWIFT_SYMBOL("s:8Foo3BarV") Bar final {
public:
  SWIFT_INLINE_THUNK ~Bar() noexcept {
    auto metadata = _impl::$s8Foo3BarVMa(0);
    auto *vwTableAddr = reinterpret_cast<swift::_impl::ValueWitnessTable **>(metadata._0) - 1;
#ifdef __arm64e__
    auto *vwTable = reinterpret_cast<swift::_impl::ValueWitnessTable *>(ptrauth_auth_data(reinterpret_cast<void *>(*vwTableAddr), ptrauth_key_process_independent_data, ptrauth_blend_discriminator(vwTableAddr, 11839)));
#else
    auto *vwTable = *vwTableAddr;
#endif
    vwTable->destroy(_getOpaquePointer(), metadata._0);
  }
  SWIFT_INLINE_THUNK Bar(const Bar &other) noexcept {
    auto metadata = _impl::$s8Foo3BarVMa(0);
    auto *vwTableAddr = reinterpret_cast<swift::_impl::ValueWitnessTable **>(metadata._0) - 1;
#ifdef __arm64e__
    auto *vwTable = reinterpret_cast<swift::_impl::ValueWitnessTable *>(ptrauth_auth_data(reinterpret_cast<void *>(*vwTableAddr), ptrauth_key_process_independent_data, ptrauth_blend_discriminator(vwTableAddr, 11839)));
#else
    auto *vwTable = *vwTableAddr;
#endif
    vwTable->initializeWithCopy(_getOpaquePointer(), const_cast<char *>(other._getOpaquePointer()), metadata._0);
  }
  SWIFT_INLINE_THUNK Bar &operator =(const Bar &other) noexcept {
    auto metadata = _impl::$s8Foo3BarVMa(0);
    auto *vwTableAddr = reinterpret_cast<swift::_impl::ValueWitnessTable **>(metadata._0) - 1;
#ifdef __arm64e__
    auto *vwTable = reinterpret_cast<swift::_impl::ValueWitnessTable *>(ptrauth_auth_data(reinterpret_cast<void *>(*vwTableAddr), ptrauth_key_process_independent_data, ptrauth_blend_discriminator(vwTableAddr, 11839)));
#else
    auto *vwTable = *vwTableAddr;
#endif
    vwTable->assignWithCopy(_getOpaquePointer(), const_cast<char *>(other._getOpaquePointer()), metadata._0);
  return *this;
  }
  SWIFT_INLINE_THUNK Bar &operator =(Bar &&other) = delete;
  [[noreturn]] SWIFT_INLINE_PRIVATE_HELPER Bar(Bar &&) noexcept {
  swift::_impl::_fatalError_Cxx_move_of_Swift_value_type_not_supported_yet();
  swift::_impl::_swift_stdlib_reportFatalError("swift", 5, "C++ does not support moving a Swift value yet", 45, 0);
  abort();
  }
  SWIFT_INLINE_THUNK void qux() const SWIFT_SYMBOL("s:8Foo3BarV3quxyyF");
  using Fred=Fred;
private:
  SWIFT_INLINE_THUNK Bar() noexcept {}
  static SWIFT_INLINE_THUNK Bar _make() noexcept { return Bar(); }
  SWIFT_INLINE_THUNK const char * _Nonnull _getOpaquePointer() const noexcept { return _storage; }
  SWIFT_INLINE_THUNK char * _Nonnull _getOpaquePointer() noexcept { return _storage; }

  alignas(8) char _storage[8];
  friend class _impl::_impl_Bar;
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wc++17-extensions"
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wreserved-identifier"
  typedef char $s8Foo3BarVD;
  static inline constexpr $s8Foo3BarVD __swift_mangled_name = 0;
#pragma clang diagnostic pop
#pragma clang diagnostic pop
};

Expected behavior

We expect Fred to be absent from the generated header.

Environment

Swift version 6.1-dev (LLVM f4b733c38006ec1, Swift 5c7509bef50dbd7)

Additional information

Thank you everyone working on Swift / Windows C++ interoperability... You are enabling wonderful things!