broken operator<< support in 8.x

[andijcr]

https://gcc.godbolt.org/z/bfvGf8bb4

#include <ostream>

#include "fmt/ostream.h"
#include "fmt/printf.h"

struct to_format {
    friend std::ostream& operator<<(std::ostream& os, to_format const& tf) {
        return os << "formated :" << uintptr_t(&tf);
    }
};

int main() { fmt::print("{}\n", to_format{}); } 

it seems that fmt v7 could use friend operator<< as a fallback formatter, while with v8 it fails with

<source>:11:46: warning: ignoring return value of 'std::string fmt::v8::format(fmt::v8::format_string<T ...>, T&& ...) [with T = {to_format}; std::string = std::__cxx11::basic_string<char>; fmt::v8::format_string<T ...> = fmt::v8::basic_format_string<char, to_format>]', declared with attribute 'nodiscard' [-Wunused-result]
   11 | int main() { fmt::format("{}\n", to_format{}); }
      |                                              ^
In file included from /opt/compiler-explorer/libs/fmt/trunk/include/fmt/format.h:50,
                 from /opt/compiler-explorer/libs/fmt/trunk/include/fmt/ostream.h:13,
                 from <source>:3:
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/core.h:3101:31: note: declared here
 3101 | FMT_NODISCARD FMT_INLINE auto format(format_string<T...> fmt, T&&... args)
      |                               ^~~~~~
/opt/compiler-explorer/libs/fmt/trunk/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 = to_format&; typename std::enable_if<IS_PACKED, int>::type <anonymous> = 0]':
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/core.h:1834:77:   required from 'constexpr fmt::v8::format_arg_store<Context, Args>::format_arg_store(T&& ...) [with T = {to_format&}; Context = fmt::v8::basic_format_context<fmt::v8::appender, char>; Args = {to_format}]'
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/core.h:1851:38:   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 = {to_format&}]'
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/core.h:3103:44:   required from 'std::string fmt::v8::format(fmt::v8::format_string<T ...>, T&& ...) [with T = {to_format}; std::string = std::__cxx11::basic_string<char>; fmt::v8::format_string<T ...> = fmt::v8::basic_format_string<char, to_format>]'
<source>:11:25:   required from here
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/core.h:1707:7: 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
 1707 |       formattable,
      |       ^~~~~~~~~~~
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/core.h:1707:7: note: 'formattable' evaluates to false
ASM generation compiler returned: 1

[andijcr]

my bad, i missed the FMT_DEPRECATED_OSTREAM

[jiapei-nexera]

@andijcr
Sorry, but what is your solution ?? Can you please clarify ???

[mwinterb]

Sorry, but what is your solution ?? Can you please clarify ???

If you define FMT_DEPRECATED_OSTREAM before including any fmtlib headers, fmt will use an available ostream insertion operator. https://godbolt.org/z/54a5G1qsj

The preferred mechanism is to define a formatter for a type deriving from ostream_formatter. https://fmt.dev/dev/api.html#ostream-api

[jiapei-nexera]

@mwinterb
It looks this will bring more inconvenience ??????

For instance, I'm trying to build Open3D, which has such a file TensorMap.cpp , in which there are two occurrences of fmt::format , and they seem to be 2 different formatter from the same class.

It looks it's more friendly for the end users by using the deprecated way???

[mwinterb]

It's not an issue of friendliness, the deprecated way caused problems, see #2357.
I don't know why TensorMap.cpp would cause issues, since it seems like the only types formatted in that file are int64_t and std::string, which are both intrinsically supported by fmt. Additionally, there doesn't seem to be any include of fmt/ostream.h in Open3D for the formatter to referenced in this issue to have been used.
The format function for the REHandle_abstract formatter needs to be const, which is a requirement that is enforced in more places than it was in earlier versions.

Additionally, this loop https://github.com/isl-org/Open3D/blob/174886a56cb8897ccb494272a2546c56527c65ce/cpp/open3d/t/geometry/TensorMap.cpp#L67-L68 is pointless since the return value of format is discarded, which is a warning in recent versions of fmt, and it seems as though Open3D compiles with warnings as errors.