Conflict in fmt/ostream.h

[chronoxor]

The following example does not compile under gcc version 7.3.0 (Ubuntu 7.3.0-16ubuntu3)

#include <fmt/format.h>
#include <fmt/ostream.h> // Adding this include failed with compile error!

using namespace fmt::literals;

int main(int argc, char** argv)
{
    std::cout << "{}"_format("test");
    return 0;
}

Produced error:

/home/user/CppCommon/modules/fmt/include/fmt/format.h: In instantiation of ‘constexpr const typename ParseContext::char_type* fmt::v5::internal::parse_format_specs(ParseContext&) [with T = char [5]; ParseContext = fmt::v5::basic_parse_context<char>; typename ParseContext::char_type = char]’:
/home/user/CppCommon/modules/fmt/include/fmt/format.h:2256:68:   required from ‘constexpr fmt::v5::internal::format_string_checker<Char, ErrorHandler, Args>::format_string_checker(fmt::v5::basic_string_view<Char>, ErrorHandler) [with Char = char; ErrorHandler = fmt::v5::internal::error_handler; Args = {char [5]}]’
/home/user/CppCommon/modules/fmt/include/fmt/format.h:2305:54:   required from ‘constexpr bool fmt::v5::internal::check_format_string(fmt::v5::basic_string_view<Char>, ErrorHandler) [with Char = char; ErrorHandler = fmt::v5::internal::error_handler; Args = {char [5]}]’
/home/user/CppCommon/modules/fmt/include/fmt/format.h:3850:58:   required from ‘std::__cxx11::basic_string<_CharT> fmt::v5::internal::udl_formatter<Char, CHARS>::operator()(const Args& ...) const [with Args = {char [5]}; Char = char; Char ...CHARS = {'{', '}'}]’
/home/user/CppCommon/examples/string_format.cpp:8:36:   required from here
/home/user/CppCommon/modules/fmt/include/fmt/format.h:2246:50: error: ambiguous template instantiation for ‘struct fmt::v5::formatter<char [5], char, void>’
   formatter<T, typename ParseContext::char_type> f{};
                                                  ^
/home/user/CppCommon/modules/fmt/include/fmt/format.h:3183:8: note: candidates are: template<class T, class Char> struct fmt::v5::formatter<T, Char, typename std::enable_if<fmt::v5::internal::format_type<typename fmt::v5::buffer_context<Char>::type, T>::value>::type> [with T = char [5]; Char = char]
 struct formatter<
        ^~~~~~~~~~
     T, Char,
     ~~~~~~~~
     typename std::enable_if<internal::format_type<
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
         typename buffer_context<Char>::type, T>::value>::type> {
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /home/user/CppCommon/examples/string_format.cpp:2:0:
/home/user/CppCommon/modules/fmt/include/fmt/ostream.h:113:8: note:                 template<class T, class Char> struct fmt::v5::formatter<T, Char, typename std::enable_if<fmt::v5::internal::is_streamable<T, Char>::value>::type> [with T = char [5]; Char = char]
 struct formatter<T, Char,
        ^~~~~~~~~~~~~~~~~~
     typename std::enable_if<internal::is_streamable<T, Char>::value>::type>
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /home/user/CppCommon/examples/string_format.cpp:1:0:
/home/user/CppCommon/modules/fmt/include/fmt/format.h:2246:50: error: ‘fmt::v5::formatter<char [5], char, void> f’ has incomplete type
   formatter<T, typename ParseContext::char_type> f{};
                                                  ^

[chronoxor]

Also there is a small warning/error in Clang in pedantic compile mode:

/Users/travis/build/chronoxor/CppCommon/modules/fmt/include/fmt/format.h:3885:55: error: string literal operator templates are a GNU extension [-Werror,-Wgnu-string-literal-operator-template]
FMT_CONSTEXPR internal::udl_formatter<Char, CHARS...> operator""_format() {
                                                      ^
In file included from /Users/travis/build/chronoxor/CppCommon/modules/fmt/src/posix.cc:13:
In file included from /Users/travis/build/chronoxor/CppCommon/modules/fmt/include/fmt/posix.h:28:
/Users/travis/build/chronoxor/CppCommon/modules/fmt/include/fmt/format.h:3885:55: error: string literal operator templates are a GNU extension [-Werror,-Wgnu-string-literal-operator-template]
FMT_CONSTEXPR internal::udl_formatter<Char, CHARS...> operator""_format() {
                                                      ^

[vitaut]

Fixed in 8b9fb9f, thanks for reporting! The warning should already be suppressed in https://github.com/fmtlib/fmt/blob/master/CMakeLists.txt#L101, but a PR is welcome if there is some case where it still occurs.