std::expected link error

[andrea-cassioli-maersk]

Compiling the following example

#include <expected>
#include <iostream>

int main() {
  std::expected<int, int> a;
  auto& x = a.value();
  std::cout << x << std::endl;
  return 0;
}

with

/opt/homebrew/opt/llvm@19/bin/clang++ -std=c++23 test-value.cpp

I get the following error

/opt/homebrew/opt/llvm@19/bin/clang++ -std=c++23 test-value.cpp 
Undefined symbols for architecture arm64:
  "std::__1::bad_expected_access<void>::what() const", referenced from:
      vtable for std::__1::bad_expected_access<int> in test-value-22e761.o
  "typeinfo for std::__1::bad_expected_access<void>", referenced from:
      typeinfo for std::__1::bad_expected_access<int> in test-value-22e761.o
  "vtable for std::__1::bad_expected_access<void>", referenced from:
      std::__1::bad_expected_access<void>::bad_expected_access[abi:ne190107]() in test-value-22e761.o
   NOTE: a missing vtable usually means the first non-inline virtual member function has no definition.
ld: symbol(s) not found for architecture arm64
clang++: error: linker command failed with exit code 1 (use -v to see invocation)

some more info

➜  tmp /opt/homebrew/opt/llvm@19/bin/clang++ -v
Homebrew clang version 19.1.7
Target: arm64-apple-darwin23.6.0
Thread model: posix
InstalledDir: /opt/homebrew/Cellar/llvm/19.1.7_1/bin
Configuration file: /opt/homebrew/etc/clang/arm64-apple-darwin23.cfg
System configuration file directory: /opt/homebrew/etc/clang
User configuration file directory: /Users/xxxxxxx/.config/clang

Compiling with clang version 18 gives no error.

Not sure what is the issue. Hope someone can help.

[frederick-vs-ja]

#87390 seems related.

[andrea-cassioli-maersk]

Not sure it helps, but

• using Apple clang works
• using any level of optimization (-01 and above) works, just -O0 leads to an error

[philnik777]

The problem is that you're using a libc++ dylib which is older than the headers. You need to either make sure that you're loading the correct (homebrew-provided) dylib, or have availability annotations enabled that match AppleClangs. Fundamentally this is a homebrew issue though, since they're not automagically loading the correct dylib. Please ask them for further support/guidance. I'm quite certain this isn't the first issue I've seen related to this. If they need help properly distributing Clang and libc++ they are free to ask.

[GuillaumeQuenneville]

I just ran into this issue. As @philnik777 said, it seems that it was picking up another libc++.
Adding this to my CmakeLists

if(APPLE)
    target_link_options(expected_test PRIVATE
      -L/opt/homebrew/opt/llvm/lib/c++ # TODO: non-hardcoded path
    )
endif()

fixes the issue.

[ldionne]

I agree with @philnik777 here, I think this is a Homebrew issue. If you want to file an issue against Homebrew, feel free to ping me on it and I can help them to figure out the right way to distribute on Apple platforms.

[andrea-cassioli-maersk]

Thanks @ldionne and @GuillaumeQuenneville. It is indeed a configuration issue. For future reference the issue is addressed here https://github.com/orgs/Homebrew/discussions/5849