`zig c++ -std=c++23` with `#include <iostream> + <expected>` is broken

[stackmystack]

Zig Version

0.15.1

Steps to Reproduce and Observed Behavior

Hello,

I am using zig build to build a C++ project.

Here's a minimal example:

#include <expected>
#include <string>
#include <iostream>

std::expected<int, std::string> parse_int(const std::string& s) {
    if (s.empty()) {
        return std::unexpected("empty string");
    }
    return 42;
}

int main() {
    auto result = parse_int("42");
    std::cout << "result = " << result.value() << "\n";
    return 0;
}

Invoking zig c++:

$ zig c++ -std=c++23 main.cpp
main.cpp:7:21: error: reference to 'unexpected' is ambiguous
    7 |         return std::unexpected("empty string");
      |                     ^
/Users/firas/.local/share/mise/installs/zig/0.15.1/lib/libcxx/include/__expected/unexpected.h:60:7: note: candidate found by name lookup is 'std::__1::unexpected'
   60 | class unexpected {
      |       ^
/Users/firas/.local/share/mise/installs/zig/0.15.1/lib/libcxx/include/__exception/operations.h:24:49: note: candidate found by name lookup is 'std::unexpected'
   24 | [[__noreturn__]] _LIBCPP_EXPORTED_FROM_ABI void unexpected();
      |                                                 ^
1 error generated.

The trouble comes from #include <iostream>. The moment it's removed, the compilation works.

The real source of the issue is this definition in zig/src/libs/libcxx.zig, where -D_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS is explicitly and unconditionally passed.

So if you do:

/tmp/bug
$ zig c++ -std=c++23 -U_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS main.cpp

It works:

/tmp/bug
$ ./a.out
result = 42

Shouldn't this be set according to the flags sent to modules? I couldn't find a reasonable explanation for why it's enabled by default.

Expected Behavior

It needs to compile normally, and pretty sure without forcing -U_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS.

[alexrp]

I have no idea why this is set; it dates all the way back to the C++-based Zig compiler.

I'm not comfortable with removing it in the 0.15.x branch as there's potential for breakage, but it seems reasonable to remove this flag in master and see what happens.

[stackmystack]

Thanks.

On a side note, how frequently do you guys bump LLVM and libcxx?

(because I'm interested in things like join_with_view and they're unavailable in < v21.1.0)

[alexrp]

As a general rule, we bump LLVM (and all its components like libc++, libunwind, etc) by one major version in every major version of Zig.