Updated github CI only runs against C++11 standard

[jgopel]

When switching from Travis to github CI (roughly cba5970), it seems like the testing for standards other than C++11 was lost.

This issue is relevant because compilers behave differently at different standards. For example, with clang 11.0.0, a pedantic build of fmt succeeds in C++11 mode but fails with the following error in C++20 mode.

In file included from /Users/jonathan/work/fmt/test/compile-test.cc:16:
/Users/jonathan/work/fmt/include/fmt/compile.h:518:25: error: implicit conversion changes signedness: 'long' to 'unsigned long' [-Werror,-Wsign-conversion]
  return {f, pos + (end - str.data()) + 1, ctx.next_arg_id()};
                 ~  ~~~~^~~~~~~~~~~~
/Users/jonathan/work/fmt/include/fmt/compile.h:538:31: note: in instantiation of function template specialization 'fmt::v7::detail::parse_specs<int, char>' requested here
      constexpr auto result = parse_specs<id_type>(str, POS + 2, ID);
                              ^
/Users/jonathan/work/fmt/include/fmt/compile.h:569:17: note: in instantiation of function template specialization 'fmt::v7::detail::compile_format_string<fmt::v7::detail::type_list<int>, 0, 0, FMT_COMPILE_STRING>' requested here
        detail::compile_format_string<detail::type_list<Args...>, 0, 0>(
                ^
/Users/jonathan/work/fmt/include/fmt/compile.h:648:37: note: in instantiation of function template specialization 'fmt::v7::detail::compile<int, FMT_COMPILE_STRING, 0>' requested here
  constexpr auto compiled = detail::compile<Args...>(S());
                                    ^
/Users/jonathan/work/fmt/test/compile-test.cc:140:24: note: in instantiation of function template specialization 'fmt::v7::format<FMT_COMPILE_STRING, int, 0>' requested here
  EXPECT_EQ("42", fmt::format(FMT_COMPILE("{:x}"), 0x42));
                       ^

I think the correct fix here is to add C++ standards to the build matrix and I would be happy to submit a PR for that work, if there's interest. If that's not palatable then I would like to see the setup should return to the way it was previously with C++11 being tested in g++-4.8, C++14 being tested in gcc-6, and C++17 being tested in gcc-8.

[vitaut]

#1969

[jgopel]

As I read it, that issue is about moving to Actions. This issue is about how that move to actions changed the behavior of testing. Can you help me understand how this issue duplicates that one?

[vitaut]

The migration from Travis CI to Github Actions is still in progress and is tracked in #1969. Configuration parity is just one minor aspect of it.

[jgopel]

Thank you for the explanation. If you'd like help with that, I would be interested in contributing.

[vitaut]

If you'd like help with that, I would be interested in contributing.

Sure, a PR to add a C++17 config is welcome.