Unable to use fmt::join() with user-defined operator<<()

[YukinoHayakawa]

#include <vector>
#include <string>

#include <fmt/ostream.h>

struct foo
{
    std::string a;
};

std::ostream & operator<<(std::ostream &os, const foo &f)
{
    return os << f.a;
}

int main(int argc, char *argv[])
{
    std::vector<foo> bar;
    fmt::format("{}", fmt::join(bar.begin(), bar.end(), ", "));
}

This used to work.
Now it produces the following errors:

1>------ Build started: Project: ..., Configuration: Debug x64 ------
1>Build started Wed, 8, 28, 2019 5:21:08 PM.
1>Target InitializeBuildStatus:
1>  Touching "x64\Debug\....tlog\unsuccessfulbuild".
1>Target ResolveProjectReferences:
1>Target VcpkgTripletSelection:
1>  Using triplet "x64-windows-static" from "...\vcpkg\installed\x64-windows-static\"
1>Target ClCompile:
1>  Microsoft (R) C/C++ Optimizing Compiler Version 19.22.27905 for x64
1>  Copyright (C) Microsoft Corporation.  All rights reserved.
1>  cl /c /I"...\vcpkg\installed\x64-windows-static\include" /ZI /JMC /W3 /WX /diagnostics:column /sdl /MP /Od /D _UNICODE /D UNICODE /D _SILENCE_CXX17_NEGATORS_DEPRECATION_WARNING /D _SILENCE_CXX17_ADAPTOR_TYPEDEFS_DEPRECATION_WARNING /D _ENABLE_EXTENDED_ALIGNED_STORAGE /D _CRT_SECURE_NO_WARNINGS /Gm- /EHsc /RTC1 /MTd /GS /fp:precise /permissive- /Zc:wchar_t /Zc:forScope /Zc:inline /Zc:rvalueCast /GR /std:c++17 /Fo"x64\Debug\\" /Fd"x64\Debug\vc142.pdb" /Gd /TP /wd4250 /FC /errorReport:prompt NewFile1.cpp
1>  NewFile1.cpp
1>  ...fmt\core.h(844,1): error C2664:  'const void *fmt::v6::internal::arg_mapper<Context>::map(std::nullptr_t)': cannot convert argument 1 from 'Arg' to 'signed char'
1>  ...fmt\core.h(844,1): error C2664:         with
1>  ...fmt\core.h(844,1): error C2664:         [
1>  ...fmt\core.h(844,1): error C2664:             Context=fmt::v6::basic_format_context<std::back_insert_iterator<fmt::v6::internal::buffer<char>>,char>
1>  ...fmt\core.h(844,1): error C2664:         ]
1>  ...fmt\core.h(844,1): error C2664:         and
1>  ...fmt\core.h(844,1): error C2664:         [
1>  ...fmt\core.h(844,1): error C2664:             Arg=fmt::v6::arg_join<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<foo>>>,char>
1>  ...fmt\core.h(844,1): error C2664:         ]
1>  ...fmt\core.h(844,69): message :  No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
1>  ...fmt\core.h(1000): message :  see reference to alias template instantiation 'fmt::v6::internal::mapped_type_constant<fmt::v6::arg_join<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<_Ty>>>,char>,Context>' being compiled
1>  ...fmt\core.h(1000): message :         with
1>  ...fmt\core.h(1000): message :         [
1>  ...fmt\core.h(1000): message :             _Ty=foo,
1>  ...fmt\core.h(1000): message :             Context=fmt::v6::basic_format_context<std::back_insert_iterator<fmt::v6::internal::buffer<char>>,char>
1>  ...fmt\core.h(1000): message :         ]
1>  ...fmt\core.h(1102): message :  see reference to function template instantiation 'unsigned __int64 fmt::v6::internal::encode_types<Context,fmt::v6::arg_join<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<_Ty>>>,char>,>(void)' being compiled
1>  ...fmt\core.h(1102): message :         with
1>  ...fmt\core.h(1102): message :         [
1>  ...fmt\core.h(1102): message :             Context=fmt::v6::basic_format_context<std::back_insert_iterator<fmt::v6::internal::buffer<char>>,char>,
1>  ...fmt\core.h(1102): message :             _Ty=foo
1>  ...fmt\core.h(1102): message :         ]
1>  ...fmt\core.h(1367): message :  see reference to class template instantiation 'fmt::v6::format_arg_store<fmt::v6::basic_format_context<std::back_insert_iterator<fmt::v6::internal::buffer<char>>,char>,fmt::v6::arg_join<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<_Ty>>>,char>>' being compiled
1>  ...fmt\core.h(1367): message :         with
1>  ...fmt\core.h(1367): message :         [
1>  ...fmt\core.h(1367): message :             _Ty=foo
1>  ...fmt\core.h(1367): message :         ]
1>  ...\NewFile1.cpp(18): message :  see reference to function template instantiation 'std::basic_string<char,std::char_traits<char>,std::allocator<char>> fmt::v6::format<char[3],fmt::v6::arg_join<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<_Ty>>>,char>,char>(const S (&),fmt::v6::arg_join<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<_Ty>>>,char> &&)' being compiled
1>  ...\NewFile1.cpp(18): message :         with
1>  ...\NewFile1.cpp(18): message :         [
1>  ...\NewFile1.cpp(18): message :             _Ty=foo,
1>  ...\NewFile1.cpp(18): message :             S=char [3]
1>  ...\NewFile1.cpp(18): message :         ]
1>  ...fmt\core.h(999): error C2062:  type 'unknown-type' unexpected
1>  ...fmt\core.h(1000,46): error C2039:  'value': is not a member of '`global namespace''
1>  ...fmt\core.h(1101,7): error C3615:  constexpr function 'fmt::v6::internal::encode_types' cannot result in a constant expression
1>  ...fmt\core.h(1102): message :  failure was caused by control reaching the end of a constexpr function
1>  ...fmt\core.h(1103,62): error C2131:  expression did not evaluate to a constant
1>  ...fmt\core.h(1103,62): message :  failure was caused by a read of an uninitialized symbol
1>  ...fmt\core.h(1103,62): message :  see usage of 'types'
1>Done building target "ClCompile" in project "....vcxproj" -- FAILED.
1>
1>Done building project "....vcxproj" -- FAILED.
1>
1>Build FAILED.
1>
1>...fmt\core.h(844,1): error C2664:  'const void *fmt::v6::internal::arg_mapper<Context>::map(std::nullptr_t)': cannot convert argument 1 from 'Arg' to 'signed char'
1>...fmt\core.h(844,1): error C2664:         with
1>...fmt\core.h(844,1): error C2664:         [
1>...fmt\core.h(844,1): error C2664:             Context=fmt::v6::basic_format_context<std::back_insert_iterator<fmt::v6::internal::buffer<char>>,char>
1>...fmt\core.h(844,1): error C2664:         ]
1>...fmt\core.h(844,1): error C2664:         and
1>...fmt\core.h(844,1): error C2664:         [
1>...fmt\core.h(844,1): error C2664:             Arg=fmt::v6::arg_join<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<foo>>>,char>
1>...fmt\core.h(844,1): error C2664:         ]
1>...fmt\core.h(999): error C2062:  type 'unknown-type' unexpected
1>...fmt\core.h(1000,46): error C2039:  'value': is not a member of '`global namespace''
1>...fmt\core.h(1101,7): error C3615:  constexpr function 'fmt::v6::internal::encode_types' cannot result in a constant expression
1>...fmt\core.h(1103,62): error C2131:  expression did not evaluate to a constant
1>    0 Warning(s)
1>    5 Error(s)
1>
1>Time Elapsed 00:00:00.87
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

[vitaut]

This is a side effect of #952. fmt::join now requires the element type to be formattable (have formatter specialization).

[YukinoHayakawa]

It looks like formatter<arg_join<Iter>> derives from formatter<Iter::value_type> which is not defined using operator<< by default anymore (in ostream) due to the changes in #952. So I have to roll my own. The old behavior is moved to internal::fallback_formatter.

My workaround is:

template <typename Char>
struct fmt::formatter<foo, Char> :
    fmt::v6::internal::fallback_formatter<foo, Char> { };

As #952 (comment) suggests, probably it's better to add a trait to enable the old stream formatter by default. It would also make join() more useful as it should be.

[roman-orekhov]

I wonder why isn't it documented at the API page (together with a workaround).