clang build error with version 8.0: undefined symbol: void ... vformat_to<char>(...)

[jdrouhard]

Given the following code:

#include <fmt/format.h>
#include <vector>

void fooImpl(std::vector<char> & chars, fmt::string_view fmt, fmt::format_args args) {
    fmt::vformat_to(std::back_inserter(chars), fmt, args);
}

template <typename S, typename... T>
std::vector<char> foo(const S& s, T &&... args) {
    using namespace fmt;
    std::vector<char> val;
    fooImpl(val, to_string_view(s), make_args_checked<T...>(s, args...));
    return val;
}

int main(int argc, char ** argv) {
    auto chars = foo("testing {}: {}", argc, argv[0]);
    return 0;
}

I get:

ld.lld: error: undefined symbol: void fmt::v7::detail::vformat_to(fmt::v7::detail::buffer&, fmt::v7::basic_string_view, fmt::v7::basic_format_args<fmt::v7::basic_format_context<std::conditional<std::is_same<fmt::v7::type_identity::type, char>::value, fmt::v7::appender, std::back_insert_iterator<fmt::v7::detail::buffer<fmt::v7::type_identity::type> > >::type, fmt::v7::type_identity::type> >, fmt::v7::detail::locale_ref)

Same exact problem with ld and gold, but lld prints the demangled symbol. I know that symbol is defined:

$ nm -C -D libfmt.so.8.0.0
0000000000020d30 W void fmt::v7::detail::vformat_to(fmt::v7::detail::buffer&, fmt::v7::basic_string_view, fmt::v7::basic_format_args<fmt::v7::basic_format_context<std::conditional<std::is_same<fmt::v7::type_identity::type, char>::value, fmt::v7::appender, std::back_insert_iterator<fmt::v7::detail::buffer<fmt::v7::type_identity::type> > >::type, fmt::v7::type_identity::type> >, fmt::v7::detail::locale_ref)

Tested on clang 12 and trunk, using all 3 main linkers (ld, lld, and gold). I'm attempting to link the shared object (not using the header-only version).

GCC 8 and 9 (and presumably 10, 11, and trunk) all seem to handle this fine. clang also works fine with 7.1.3, so it's something new in 8.0.0 that it doesn't like. If it's a clang problem, I'm wondering if there's a workaround? I believe vformat_to<char> was already an extern template with an explicit instantiation in 7.1.3, so I'm not sure what else might be causing this.

[vitaut]

Looks like the error is due to incompatibility between clang and gcc. I was able to repro it on xenial as follows:

$ git clone https://github.com/fmtlib/fmt.git
$ cd fmt
$ CXX=clang++-8 cmake -DBUILD_SHARED_LIBS=TRUE .
$ make -j10
$ c++ path/to/test.cc -lfmt -L. -I include -std=c++11

/tmp/ccnVNvdV.o: In function `std::back_insert_iterator<std::vector<char, std::allocator<char> > > fmt::v8::vformat_to<std::back_insert_iterator<std::vector<char, std::allocator<char> > >, 0>(std::back_insert_iterator<std::vector<char, std::allocator<char> > >, fmt::v8::basic_string_view<char>, fmt::v8::basic_format_args<fmt::v8::basic_format_context<fmt::v8::appender, char> >)':
test.cc:(.text._ZN3fmt2v810vformat_toISt20back_insert_iteratorISt6vectorIcSaIcEEELi0EEET_S7_NS0_17basic_string_viewIcEENS0_17basic_format_argsINS0_20basic_format_contextINS0_8appenderEcEEEE[_ZN3fmt2v810vformat_toISt20back_insert_iteratorISt6vectorIcSaIcEEELi0EEET_S7_NS0_17basic_string_viewIcEENS0_17basic_format_argsINS0_20basic_format_contextINS0_8appenderEcEEEE]+0x63): undefined reference to `void fmt::v8::detail::vformat_to<char>(fmt::v8::detail::buffer<char>&, fmt::v8::basic_string_view<char>, fmt::v8::basic_format_args<fmt::v8::basic_format_context<std::conditional<std::is_same<fmt::v8::type_identity<char>::type, char>::value, fmt::v8::appender, std::back_insert_iterator<fmt::v8::detail::buffer<fmt::v8::type_identity<char>::type> > >::type, fmt::v8::type_identity<char>::type> >, fmt::v8::detail::locale_ref)'

[vitaut]

Should be fixed in 3e7a29c. Thanks for reporting.

[sangelovic]

Just for the record, I got the same linking errors with Clang 14.0.3 while it's working with gcc. I already am using fmtlib v8.1.1, which should have fixed that. After some investigations, I found the cause: I included core.h header file: #include <fmt/core.h>. Replacing that with #include <fmt/format.h> fixed the problem. Hope it helps others experiencing the same issue.