build error when format boost::filesystem::path using 8.1.1

[chengfzy]

Building error for below code when using fmt 8.1.1, enen I add <fmt/ostream.h>

#include <fmt/ostream.h>
#include <fmt/ranges.h>
#include <boost/filesystem.hpp>
#include <iostream>

using namespace std;
using namespace fmt;

int main(int argc, char* argv[]) {
    boost::filesystem::path p("./data");
    cout << "print using cout, p = " << p << endl;  // OK
    cout << format("p = {}", p.string()) << endl;   // OK
    cout << format("p = {}", p) << endl;            // error
    return 0;
}

The output is:

[proc] Executing command: /usr/local/bin/cmake --build /home/jeffery/Documents/Code/Others/CppTemp/buildDebug --config Debug --target all -j 4 --
[build] [1/2  50% :: 1.667] Building CXX object CMakeFiles/CppTemp.dir/src/main.cpp.o
[build] FAILED: CMakeFiles/CppTemp.dir/src/main.cpp.o 
[build] /bin/x86_64-linux-gnu-g++-9 -DBOOST_ALL_NO_LIB -DBOOST_FILESYSTEM_DYN_LINK -DFMT_HEADER_ONLY=1 -DGFLAGS_IS_A_DLL=0 -DGTEST_LINKED_AS_SHARED_LIBRARY=1 -DJSON_DIAGNOSTICS=0 -DJSON_USE_IMPLICIT_CONVERSIONS=1 -I../include -isystem /usr/local/include/eigen3 -isystem /usr/local/include/opencv4 -g -fPIE -std=gnu++17 -MD -MT CMakeFiles/CppTemp.dir/src/main.cpp.o -MF CMakeFiles/CppTemp.dir/src/main.cpp.o.d -o CMakeFiles/CppTemp.dir/src/main.cpp.o -c ../src/main.cpp
[build] In file included from /usr/local/include/fmt/format.h:48,
[build]                  from /usr/local/include/fmt/ostream.h:13,
[build]                  from ../src/main.cpp:1:
[build] /usr/local/include/fmt/core.h: In substitution of ‘template<bool B> using bool_constant = std::integral_constant<bool, __v> [with bool B = ((fmt::v8::detail::has_const_formatter<boost::filesystem::path, fmt::v8::basic_format_context<fmt::v8::appender, char> >() || (!(bool)std::integral_constant<bool, false>::value)) || ((bool)std::integral_constant<bool, true>::value))]’:
[build] /usr/local/include/fmt/core.h:1450:10:   required from ‘struct fmt::v8::detail::arg_mapper<fmt::v8::basic_format_context<fmt::v8::appender, char> >::formattable<boost::filesystem::path, boost::filesystem::path>’
[build] /usr/local/include/fmt/core.h:1465:25:   required by substitution of ‘template<class T, typename std::enable_if<(! fmt::v8::detail::arg_mapper<fmt::v8::basic_format_context<fmt::v8::appender, char> >::formattable<T, typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type>::value), int>::type <anonymous> > constexpr fmt::v8::detail::unformattable_const fmt::v8::detail::arg_mapper<fmt::v8::basic_format_context<fmt::v8::appender, char> >::do_map<T, <enumerator> >(T&&) [with T = boost::filesystem::path; typename std::enable_if<(! fmt::v8::detail::arg_mapper<fmt::v8::basic_format_context<fmt::v8::appender, char> >::formattable<T, typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type>::value), int>::type <anonymous> = <missing>]’
[build] /usr/local/include/fmt/core.h:1476:33:   required by substitution of ‘template<class T, class U, typename std::enable_if<((((! fmt::v8::detail::is_string<U>::value) && (! fmt::v8::is_char<U>::value)) && (! std::is_array<_To>::value)) && (std::is_constructible<fmt::v8::formatter<U, char, void> >::value || std::is_constructible<fmt::v8::detail::fallback_formatter<U, char, void> >::value)), int>::type <anonymous> > constexpr decltype (((fmt::v8::detail::arg_mapper<fmt::v8::basic_format_context<fmt::v8::appender, char> >*)this)->fmt::v8::detail::arg_mapper<fmt::v8::basic_format_context<fmt::v8::appender, char> >::do_map(forward<T>(val))) fmt::v8::detail::arg_mapper<fmt::v8::basic_format_context<fmt::v8::appender, char> >::map<T, U, <enumerator> >(T&&) [with T = boost::filesystem::path; U = boost::filesystem::path; typename std::enable_if<((((! fmt::v8::detail::is_string<U>::value) && (! fmt::v8::is_char<U>::value)) && (! std::is_array<_To>::value)) && (std::is_constructible<fmt::v8::formatter<U, char, void> >::value || std::is_constructible<fmt::v8::detail::fallback_formatter<U, char, void> >::value)), int>::type <anonymous> = 0]’
[build] /usr/local/include/fmt/core.h:1806:5:   required by substitution of ‘template<class FormatContext, class U, typename std::enable_if<std::is_same<U, const boost::filesystem::path>::value, int>::type <anonymous> > decltype (ctx.out()) fmt::v8::formatter<boost::filesystem::path, char, void>::format<FormatContext, U, <enumerator> >(U&, FormatContext&) [with FormatContext = fmt::v8::basic_format_context<fmt::v8::appender, char>; U = const boost::filesystem::path; typename std::enable_if<std::is_same<U, const boost::filesystem::path>::value, int>::type <anonymous> = 0]’
[build] /usr/local/include/fmt/core.h:730:72:   required by substitution of ‘template<class Context, class T> constexpr decltype ((typename Context::formatter_type<T>().format(declval<const T&>(), declval<Context&>()), true)) fmt::v8::detail::has_const_formatter_impl(T*) [with Context = fmt::v8::basic_format_context<fmt::v8::appender, char>; T = boost::filesystem::path]’
[build] /usr/local/include/fmt/core.h:740:43:   [ skipping 4 instantiation contexts, use -ftemplate-backtrace-limit=0 to disable ]
[build] /usr/local/include/fmt/core.h:1492:7:   required by substitution of ‘template<class T, class Context> using mapped_type_constant = fmt::v8::detail::type_constant<decltype (fmt::v8::detail::arg_mapper<Context>().map(declval<const T&>())), typename Context::char_type> [with T = boost::filesystem::path; Context = fmt::v8::basic_format_context<fmt::v8::appender, char>]’
[build] /usr/local/include/fmt/core.h:1688:10:   required from ‘constexpr long long unsigned int fmt::v8::detail::encode_types() [with Context = fmt::v8::basic_format_context<fmt::v8::appender, char>; Arg = boost::filesystem::path; Args = {}]’
[build] /usr/local/include/fmt/core.h:1840:58:   required from ‘constexpr const long long unsigned int fmt::v8::format_arg_store<fmt::v8::basic_format_context<fmt::v8::appender, char>, boost::filesystem::path>::desc’
[build] /usr/local/include/fmt/core.h:1954:45:   required from ‘constexpr fmt::v8::basic_format_args<Context>::basic_format_args(const fmt::v8::format_arg_store<Context, Args ...>&) [with Args = {boost::filesystem::path}; Context = fmt::v8::basic_format_context<fmt::v8::appender, char>]’
[build] /usr/local/include/fmt/core.h:3119:17:   required from ‘std::string fmt::v8::format(fmt::v8::format_string<T ...>, T&& ...) [with T = {boost::filesystem::path&}; std::string = std::__cxx11::basic_string<char>; fmt::v8::format_string<T ...> = fmt::v8::basic_format_string<char, boost::filesystem::path&>]’
[build] ../src/main.cpp:13:31:   required from here
[build] /usr/local/include/fmt/core.h:1451:54: error: ‘constexpr bool fmt::v8::detail::has_const_formatter() [with T = boost::filesystem::path; Context = fmt::v8::basic_format_context<fmt::v8::appender, char>]’ called in a constant expression before its definition is complete
[build]  1451 |       : bool_constant<has_const_formatter<U, Context>() ||
[build]       |                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
[build] /usr/local/include/fmt/core.h:1452:68: note: in template argument for type ‘bool’
[build]  1451 |       : bool_constant<has_const_formatter<U, Context>() ||
[build]       |                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~          
[build]  1452 |                       !std::is_const<remove_reference_t<T>>::value ||
[build]       |                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
[build]  1453 |                       has_fallback_formatter<U, char_type>::value> {};
[build]       |                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~   
[build] ninja: build stopped: subcommand failed.
[build] Build finished with exit code 1

[vitaut]

In a way this is addressed in the current master by requiring a formatter specialization for types with operator<< (https://godbolt.org/z/7sTz3Yo7W):

template <>
struct fmt::formatter<boost::filesystem::path> : ostream_formatter {};

It's a bit unfortunate that ostream formatting is broken for boost::filesystem::path but I don't think it's worth implementing a workaround considering that implicit ostream insertion operator support is going away.