Update fmt lib to v9.0.0

[YuZhangGHub]

Hi fmt v9.0.0 has a big update and better implementation on snprintf and output. Please update it. Thanks.

[xbreak]

Note that fmt v9.0.0 deprecated automatic ostream formatting that may need additional consideration by spdlog (I'm currently evaluating spdlog v1.10.0 with fmt v9.1.0):

Disabled automatic std::ostream insertion operator (operator<<) discovery when fmt/ostream.h is included to prevent ODR violations. You can get the old behavior by defining FMT_DEPRECATED_OSTREAM but this will be removed in the next major release. Use fmt::streamed or fmt::ostream_formatter to enable formatting via std::ostream instead.

https://github.com/fmtlib/fmt/releases/tag/9.0.0

[gabime]

Thanks for the heads up. what additional consideration is needed?

[xbreak]

I'm not sure if you or your users would consider it a breaking change in spdlog if automatic ostream formatting disappears for example.

[offa]

An argument for the update is that fmt v9.x fixes a deprecation warning on newer Clang version with C++20 (fmtlib/fmt#2408).

[SX91]

I'm not sure if you or your users would consider it a breaking change in spdlog if automatic ostream formatting disappears for example.

The old behaviour could be activated with FMT_DEPRECATED_OSTREAM definition. It would be removed in future major fmtlib releases (as stated in release notes for 9.0.0). Yet it still could be used as a workaround to introduce 9.x.x support without breaking backward compatibility.

[gabime]

Thanks for the info @SX91 !
For the upcoming spdlog release I will have to consider this (whether to define FMT_DEPRECATED_OSTREAM or not).

[SX91]

Hello, @gabime ! Thanks for your reply.

I think, leaving this problem to end user would break existing code, which relies on auto ostream-based formatting.
Sure, user could add this definition manually on cmake level, but it would be nice to have an option to disable deprecated behaviour (enabled by default) via cmake option or tweakme.h. At least until next major fmtlib upgrade.

If there's no other backwards incompatible changes in spdlog that is.

[gabime]

Updated to fmt version 9.1.0. Preserved for this version the deprecated ostream formatting by defining FMT_DEPRECATED_OSTREAM