Visual Studio 22 (17.2.3) + Module compilation results in ambiguous format_to call

[Agga]

Setup

# CMakeLists.txt

set(FMT_CAN_MODULE OFF)
if (CMAKE_CXX_STANDARD GREATER 17 AND
    # msvc 16.10-pre4
    MSVC AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 19.29.30035)
  set(FMT_CAN_MODULE ON) # << this is a modification from me to get module compilation going
endif ()

cmake -DCMAKE_CXX_STANDARD=20 -DFMT_MODULE=ON

Bug

The format_to call in os.cc function format_windows_error becomes ambiguous.

fmt/src/os.cc

Line 172 in 9d60395

format_to(buffer_appender<char>(out), "{}: {}", message, utf8_message);

2>fmt.cc
2>D:\projects\fmt-master\src\os.cc(172,78): error C2668: 'fmt::v8::format_to': ambiguous call to overloaded function
2>D:\projects\fmt-master\include\fmt\core.h(3139,17): message : could be 'OutputIt fmt::v8::format_to<fmt::v8::appender,const char*&,fmt::v8::detail::utf16_to_utf8&,0>(OutputIt,fmt::v8::basic_format_string<char,const char *&,fmt::v8::detail::utf16_to_utf8 &>,const char *&,fmt::v8::detail::utf16_to_utf8 &)'
2>        with
2>        [
2>            OutputIt=fmt::v8::appender
2>        ]
2>C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.32.31326\include\format(3046,11): message : or       '_OutputIt std::format_to<fmt::v8::appender,const char*&,fmt::v8::detail::utf16_to_utf8&>(_OutputIt,const std::_Basic_format_string<char,const char *&,fmt::v8::detail::utf16_to_utf8 &>,const char *&,fmt::v8::detail::utf16_to_utf8 &)' [found using argument-dependent lookup]
2>        with
2>        [
2>            _OutputIt=fmt::v8::appender
2>        ]
2>D:\projects\fmt-master\src\os.cc(172,78): message : while trying to match the argument list '(fmt::v8::appender, const char [7], const char *, fmt::v8::detail::utf16_to_utf8)'
2>Generating Code...
2>Done building project "fmt.vcxproj" -- FAILED.
========== Build: 1 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Possible Explanation

fmt.cc includes <chrono> which pulls in <format>. Which turns this https://en.cppreference.com/w/cpp/utility/format/format_to into a valid candidate and makes the call ambiguous.

[vitaut]

Please post the complete error message.

[Agga]

added error message

[vitaut]

Looks like a bug in MSVC: std::format_to shouldn't be discoverable via ADL because none of the arguments are in the std namespace. Please report to Microsoft.

[mwinterb]

Does this issue not reduce down to this since the base class of appender is std::back_insert_iterator? https://gcc.godbolt.org/z/EWaG3G7GG

[Agga]

buffer_appender<char>(out)

I think this is actually a std::back_inserter object - that should pull std::format_to into the lookup resolution?

fmt/include/fmt/core.h

Line 1448 in 9d60395

class appender : public std::back_insert_iterator<detail::buffer<char>> {

fmt/include/fmt/core.h

Line 955 in 9d60395

using buffer_appender = conditional_t<std::is_same<T, char>::value, appender,

[vitaut]

Ah, good catch. In that case we should namespace qualify the call to format_to, i.e. change it to fmt::format_to. A PR is welcome!