Compile issues with libfmt >=8.1.0

[blattms]

Seems like newer versions are less forgiving when formatting enum classes.

/usr/bin/c++ -DBOOST_ALL_NO_LIB -DBOOST_SYSTEM_DYN_LINK -DEMBEDDED_PYTHON -DFMT_LOCALE -DFMT_SHARED -DHAVE_CONFIG_H=1 -Dopmcommon_EXPORTS -I/build/opm-common-2021.10/obj-x86_64-linux-gnu -I/build/opm-common-2021.10/obj-x86_64-linux-gnu/include -I/build/opm-common-2021.10 -I/usr/include/cjson -isystem /usr/include/python3.9 -g -O2 -ffile-prefix-map=/build/opm-common-2021.10=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -pipe -fopenmp -pthread -fPIC -fopenmp -std=c++17 -MD -MT CMakeFiles/opmcommon.dir/src/opm/parser/eclipse/EclipseState/Schedule/UDQ/UDQState.cpp.o -MF CMakeFiles/opmcommon.dir/src/opm/parser/eclipse/EclipseState/Schedule/UDQ/UDQState.cpp.o.d -o CMakeFiles/opmcommon.dir/src/opm/parser/eclipse/EclipseState/Schedule/UDQ/UDQState.cpp.o -c /build/opm-common-2021.10/src/opm/parser/eclipse/EclipseState/Schedule/UDQ/UDQState.cpp
In file included from /usr/include/fmt/format.h:48,
                 from /build/opm-common-2021.10/src/opm/parser/eclipse/EclipseState/Schedule/UDQ/UDQSet.cpp:22:
/usr/include/fmt/core.h: In instantiation of 'constexpr fmt::v8::detail::value<Context> fmt::v8::detail::make_arg(T&&) [with bool IS_PACKED = true; Context = fmt::v8::basic_format_context<fmt::v8::appender, char>; fmt::v8::detail::type <anonymous> = fmt::v8::detail::type::custom_type; T = Opm::UDQVarType&; typename std::enable_if<IS_PACKED, int>::type <anonymous> = 0]':
/usr/include/fmt/core.h:1855:77:   required from 'constexpr fmt::v8::format_arg_store<Context, Args>::format_arg_store(T&& ...) [with T = {const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, long unsigned int&, Opm::UDQVarType&, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, long unsigned int&, Opm::UDQVarType&}; Context = fmt::v8::basic_format_context<fmt::v8::appender, char>; Args = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, long unsigned int, Opm::UDQVarType, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, long unsigned int, Opm::UDQVarType}]'
/usr/include/fmt/core.h:1872:38:   required from 'constexpr fmt::v8::format_arg_store<Context, typename std::remove_cv<typename std::remove_reference<Args>::type>::type ...> fmt::v8::make_format_args(Args&& ...) [with Context = fmt::v8::basic_format_context<fmt::v8::appender, char>; Args = {const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, long unsigned int&, Opm::UDQVarType&, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, long unsigned int&, Opm::UDQVarType&}]'
/usr/include/fmt/core.h:3119:44:   required from 'std::string fmt::v8::format(fmt::v8::format_string<T ...>, T&& ...) [with T = {const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, long unsigned int, Opm::UDQVarType, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, long unsigned int, Opm::UDQVarType}; std::string = std::__cxx11::basic_string<char>; fmt::v8::format_string<T ...> = fmt::v8::basic_format_string<char, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, long unsigned int, Opm::UDQVarType, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, long unsigned int, Opm::UDQVarType>]'
/build/opm-common-2021.10/src/opm/parser/eclipse/EclipseState/Schedule/UDQ/UDQSet.cpp:523:27:   required from here
/usr/include/fmt/core.h:1728:7: error: static assertion failed: Cannot format an argument. To make type T formattable provide a formatter<T> specialization: https://fmt.dev/latest/api.html#udt
 1728 |       formattable,
      |       ^~~~~~~~~~~
/usr/include/fmt/core.h:1728:7: note: 'formattable' evaluates to false

The problem is that enum classes are not anymore cast internally to int (or similar). See comment on issue 391 of fmt, issue /fmtlib/fmt#1841, and commit fd62fba985.
We either need to provide a formatter, cast to int when formatting, or provide operator<< for all enum classes used with format. To me the latter seems most intriguing.

[blattms]

87 enum classes in opm-common. Wondering how many are used in fmt::format. Guess I'll need to try.

[atgeirr]

We have quite a few "enum to string" facilities in opm-common, is this better done with operator<< instead? Is the opposite operator possible to define as well, for "string to enum" conversion?

[bska]

We have quite a few "enum to string" facilities in opm-common, is this better done with operator<< instead?

I don't really see how we can define an operator<<() overload that converts an enum into a string with no other context. Those output operators typically take a std::basic_ostream<>& as their first argument and are specifically intended to write values into an output stream.

[atgeirr]

I don't really see how we can define an operator<<() overload that converts an enum into a string with no other context. Those output operators typically take a std::basic_ostream<>& as their first argument and are specifically intended to write values into an output stream.

I was implicitly assuming the facility was mostly used for logging, when we often have a stream, but that may not be the case? The distinction between doing stream insertion and string conversion is always annoying me...

[bska]

I don't really see how we can define an operator<<() overload that converts an enum into a string with no other context. Those output operators typically take a std::basic_ostream<>& as their first argument and are specifically intended to write values into an output stream.

I was implicitly assuming the facility was mostly used for logging, when we often have a stream, but that may not be the case? The distinction between doing stream insertion and string conversion is always annoying me...

I think operator<<() would work absolutely fine in the context of logging, but we also have a great many locations that use

throw some_exception_type {
    "Unsupported option value '" + stringify(enum_value) + '\''
};

and that's more difficult to handle with operator<<(). Of course if we were more disciplined in our error handling...