[C++20][Modules] Can't import <complex> and <iostream> at the same time with libc++

[Arthapz]

This snippet fails to build on df34581 with https://reviews.llvm.org/D142704 applied

import <iostream>;
import <complex>;

using namespace std;

int main(int argc, char** argv)
{
    cout << "hello world!" << endl;
    return 0;
}

/usr/bin/clang -Qunused-arguments -m64 -fvisibility=hidden -fvisibility-inlines-hidden -O3 -std=c++20 -fmodules -fno-implicit-module-maps -stdlib=libc++ -DNDEBUG -fmodules-cache-path=build/stlmodules/cache -c -Wno-everything -o build/stlmodules/cache/iostream.pcm -x c++-system-header iostream

/usr/bin/clang -Qunused-arguments -m64 -fvisibility=hidden -fvisibility-inlines-hidden -O3 -std=c++20 -fmodules -fno-implicit-module-maps -stdlib=libc++ -DNDEBUG -fmodules-cache-path=build/stlmodules/cache -c -Wno-everything -o build/stlmodules/cache/complex.pcm -x c++-system-header complex

/usr/bin/clang -c -Qunused-arguments -m64 -fvisibility=hidden -fvisibility-inlines-hidden -O3 -std=c++20 -fmodules -fno-implicit-module-maps -stdlib=libc++ -DNDEBUG -fmodule-file=build/stlmodules/cache/iostream.pcm -fmodule-file=build/stlmodules/cache/complex.pcm -o build/.objs/stl_headerunit_cpp_only/linux/x86_64/release/src/main.cpp.o src/main.cpp
error: /usr/bin/../include/c++/v1/ostream:254:20: error: 'std::basic_ostream<char>::operator<<' from module '/usr/bin/../include/c++/v1/complex' is not present in definition of 'std::ostream' in module '/usr/bin/../include/c++/v1/iostream'
    basic_ostream& operator<<(basic_streambuf<char_type, traits_type>* __sb);
                   ^
/usr/bin/../include/c++/v1/ostream:221:20: note: declaration of 'operator<<' does not match
    basic_ostream& operator<<(basic_ostream& (*__pf)(basic_ostream&))
                   ^
/usr/bin/../include/c++/v1/ostream:225:20: note: declaration of 'operator<<' does not match
    basic_ostream& operator<<(basic_ios<char_type, traits_type>&
                   ^
/usr/bin/../include/c++/v1/ostream:230:20: note: declaration of 'operator<<' does not match
    basic_ostream& operator<<(ios_base& (*__pf)(ios_base&))
                   ^
/usr/bin/../include/c++/v1/ostream:233:20: note: declaration of 'operator<<' does not match
    basic_ostream& operator<<(bool __n);
                   ^
/usr/bin/../include/c++/v1/ostream:234:20: note: declaration of 'operator<<' does not match
    basic_ostream& operator<<(short __n);

[llvmbot]

@llvm/issue-subscribers-clang-modules

[Arthapz]

May be the same as #58540

[koplas]

The diagnostic message is misleading. All member functions clang complains about are in both header units and even the AST matches. The preprocessor output also reveals no big differences. There seems to be a problem with the merging.

Update: While checking the PendingOdrMergeChecks it does not find the declarations.

[koplas]

Here is a still big repro for this issue: headerunits_repro.zip

Update:
Here is a reproducer with an interesting test: https://github.com/koplas/clang-headerunits
cvise seems only to be able to remove comments.

[koplas]

@Arthapz Can you check if this issue occurs with a clang build with this flag: -DLIBCXX_ENABLE_WIDE_CHARACTERS=OFF?

[koplas]

Here is a big reproducer:

// ostream.hpp
namespace std {
inline namespace __1 {
template <class _CharT> struct char_traits;
}
} // namespace std

namespace std {
inline namespace __1 {
class ios_base;

template <class _CharT, class _Traits = char_traits<_CharT>> class basic_ios;

template <class _CharT, class _Traits = char_traits<_CharT>>
class basic_ostream;

typedef basic_ios<char> ios;
typedef basic_ios<wchar_t> wios;

typedef basic_ostream<char> ostream;

typedef basic_ostream<wchar_t> wostream;

template <class _CharT, class _Traits>
class __attribute__((__preferred_name__(ios)))
__attribute__((__preferred_name__(wios))) basic_ios;
} // namespace __1
} // namespace std

namespace std {
inline namespace __1 {

class ios_base {
public:
  class failure;

  virtual ~ios_base();

protected:
  ios_base() {}

  void init(void *__sb);
};

template <class _CharT, class _Traits> class basic_ios : public ios_base {
public:
  typedef _Traits traits_type;

  explicit basic_ios(void *__sb);
  ~basic_ios() override;

protected:
  basic_ios() {}
  void init(void *__sb);
};

template <class _CharT, class _Traits>
inline basic_ios<_CharT, _Traits>::basic_ios(void *__sb) {
  init(__sb);
}

template <class _CharT, class _Traits>
basic_ios<_CharT, _Traits>::~basic_ios() {}

template <class _CharT, class _Traits>
inline void basic_ios<_CharT, _Traits>::init(void *__sb) {}
} // namespace __1
} // namespace std

namespace std {
inline namespace __1 {
extern template class basic_ios<char>;
}
} // namespace std

namespace std {
inline namespace __1 {

template <class _CharT, class _Traits>
class __attribute__((__type_visibility__("default"))) basic_ostream
    : virtual public basic_ios<_CharT, _Traits> {
public:
  inline explicit basic_ostream(void *__sb) { this->init(__sb); }
  ~basic_ostream() override;

protected:
  inline basic_ostream(basic_ostream &&__rhs);

protected:
  basic_ostream() {}
};
} // namespace __1
} // namespace std

// a.hpp
#include "ostream.hpp"

// b.hpp
#include "ostream.hpp"

// main.cpp
import "a.hpp";
import "b.hpp";


std::basic_ostream<char> test()
{
    return std::basic_ostream<char>(nullptr);
}

clang++ -stdlib=libc++ -std=c++20 -xc++-user-header --precompile a.hpp -o a.pcm
clang++ -stdlib=libc++ -std=c++20 -xc++-user-header --precompile b.hpp -o b.pcm
clang++ -stdlib=libc++ -std=c++20 -fmodule-file=a.pcm -fmodule-file=b.pcm -c main.cpp

Output:

In module '/home/xxx/Documents/clang_header_units_repro/a.hpp':
/home/xxx/Documents/clang_header_units_repro/ostream.hpp:49:3: error: 'std::basic_ios<char>::~basic_ios' from module '/home/xxx/Documents/clang_header_units_repro/a.hpp' is not present in definition of 'std::ios' in module '/home/xxx/Documents/clang_header_units_repro/b.hpp'
  ~basic_ios() override;
  ^
/home/xxx/Documents/clang_header_units_repro/ostream.hpp:71:23: note: definition has no member '~basic_ios'
extern template class basic_ios<char>;
                      ^
In module '/home/xxx/Documents/clang_header_units_repro/a.hpp':
/home/xxx/Documents/clang_header_units_repro/ostream.hpp:48:12: error: 'std::basic_ios<char>::basic_ios' from module '/home/xxx/Documents/clang_header_units_repro/a.hpp' is not present in definition of 'std::ios' in module '/home/xxx/Documents/clang_header_units_repro/b.hpp'
  explicit basic_ios(void *__sb);
           ^
/home/xxx/Documents/clang_header_units_repro/ostream.hpp:71:23: note: definition has no member 'basic_ios'
extern template class basic_ios<char>;
                      ^
In module '/home/xxx/Documents/clang_header_units_repro/a.hpp':
/home/xxx/Documents/clang_header_units_repro/ostream.hpp:52:3: error: 'std::basic_ios<char>::basic_ios' from module '/home/xxx/Documents/clang_header_units_repro/a.hpp' is not present in definition of 'std::ios' in module '/home/xxx/Documents/clang_header_units_repro/b.hpp'
  basic_ios() {}
  ^
/home/xxx/Documents/clang_header_units_repro/ostream.hpp:71:23: note: definition has no member 'basic_ios'
extern template class basic_ios<char>;
                      ^
3 errors generated.

Note: For brevity, this reproducer highlights the issue only with the constructor and no other functions, like the streaming operators. But the root cause should be the same.