Regression? undefined reference when using different compiler

[Phygon]

I just tried to upgrade from fmt 6.2.1 to fmt 7.0.0.
I compiled fmt with the default compiler (GCC 8.3 on Debian Buster), while the application is compiled with the most current clang++.

Reduced example code:

#include <fmt/format.h>
int main()
{
        fmt::memory_buffer mb;
        fmt::format_to(mb, "Hello {}!", 42);
}

Compiled with:
clang++ -std=c++14 -o test test.cpp -lfmt

This results in the following error:

/usr/bin/ld: /tmp/test-cbfe70.o: in function `fmt::v6::basic_format_context<std::back_insert_iterator<fmt::v6::detail::buffer<char> >, char>::iterator fmt::v6::format_to<char [10], int, 500ul, char>(fmt::v6::basic_memory_buffer<char, 500ul, std::allocator<char> >&, char const (&) [10], int&&)':
test.cpp:(.text._ZN3fmt2v69format_toIA10_cJiELm500EcEENS0_20basic_format_contextISt20back_insert_iteratorINS0_6detail6bufferIT2_EEES7_E8iteratorERNS0_19basic_memory_bufferIS7_XT1_ESaIS7_EEERKT_DpOT0_[_ZN3fmt2v69format_toIA10_cJiELm500EcEENS0_20basic_format_contextISt20back_insert_iteratorINS0_6detail6bufferIT2_EEES7_E8iteratorERNS0_19basic_memory_bufferIS7_XT1_ESaIS7_EEERKT_DpOT0_]+0x88): undefined reference to `fmt::v6::basic_format_context<std::back_insert_iterator<fmt::v6::detail::buffer<char> >, char>::iterator fmt::v6::detail::vformat_to<char>(fmt::v6::detail::buffer<char>&, fmt::v6::basic_string_view<char>, fmt::v6::basic_format_args<fmt::v6::basic_format_context<std::back_insert_iterator<fmt::v6::detail::buffer<fmt::v6::type_identity<char>::type> >, fmt::v6::type_identity<char>::type> >)'

A similar error is printed when using clang++ for the fmt library and g++ for the test. No error if the compiler matches. With fmt 6.x, this also works without error.

[SpaceIm]

I don't know if it's exactly the same issue, but I have similar undefined references with Visual Studio when fmt 7.0.0 is compiled as a shared lib and then consumed in a simple cpp executable (FMT_SHARED is defined of course).

[vitaut]

Interesting. For some reason gcc doesn't resolve the buffer_context<char> alias to basic_format_context<...> which results in a symbol mismatch.

[vitaut]

Here's a minimal repro: https://godbolt.org/z/4C2Bd8

[vitaut]

Also discussed here: https://twitter.com/vzverovich/status/1280338087579488256

[vitaut]

Worked around in fbf3b94. Thanks for reporting.

[vitaut]

@Phygon, this looks like a bug in gcc so I recommend reporting there.