spdlog `1.17.0` generates deprecation warnings with fmt `12.2.0`

[absahl]

Latest version generates deprecation warnings with fmt 12.2.0. With Visual Studio 2026 on Windows 11 Pro, I am getting following warnings (promoted to errors due to compiler setting):

'fmt::v12::fstring<DWORD>::operator fmt::v12::string_view': was declared deprecated
'fmt::v12::fstring<const std::string &>::operator fmt::v12::string_view': was declared deprecated
'fmt::v12::fstring<const std::string &>::operator fmt::v12::string_view': was declared deprecated
'fmt::v12::fstring<const std::string &>::operator fmt::v12::string_view': was declared deprecated
'fmt::v12::fstring<const size_t &,unsigned __int64,unsigned __int64>::operator fmt::v12::string_view': was declared deprecated
'fmt::v12::fstring<const DWORD &>::operator fmt::v12::string_view': was declared deprecated

I built both using vcpkg.

[tt4g]

spdlog does not support fmt v12.2.0 yet.
Pull requests are welcome.

By the way, based on the message you provided, I can't tell how this relates to spdlog.
Could you provide a message that indicates which spdlog file is generating the warning?

[tt4g]

Related changes: fmtlib/fmt@f3d510c

[tomatotomata]

Hi @tt4g, I traced the warning to the external-fmt path in include/spdlog/logger.h: the templated logger::log() passes fmt::format_string directly into log_(), whose fmt::basic_string_view parameter selects fmt 12's deprecated conversion operator. The wrapper overloads also pass that format-string object by value again.

PR #3541 used the same boundary as the fmt implementation, but that change is not present on the current v1.x branch. I was thinking of preparing a small v1.x fix that forwards the stored view (fmt.get() or fmt.str) through the logger and global wrapper overloads, then compiling with external fmt 12 and warnings-as-errors. Would that be useful for this issue, or is there a reason the branch should keep the current conversion path?

[tt4g]

@tomatotomata It was modified the next day by the same contributor: #3543
If you know how to resolve this issue, please submit a PR.

[tt4g]

I re-examined the spdlog and fmt.
Although the issue does not include a detailed trace of the warning, it appears that fmt::fstring::operator const string_view&() (deprecated in fmt v12.20 by fmtlib/fmt@f3d510c) is the source of the warning:
https://github.com/fmtlib/fmt/blob/12.2.0/include/fmt/base.h#L2728

spdlog::details::to_string_view(spdlog::string_view_t str) that called operator was removed by PR #3541.
As the contributor explains in the PR #3541, I believe this issue has already been resolved.

@tomatotomata The warning will probably not appear in the next spdlog release.

[tomatotomata]

I checked the merged follow-up in #3543 and the current v1.x header. The logger now forwards fmt.get() at the boundary that was triggering fmt::fstring::operator const string_view&(), so the fix is already present upstream and there is no separate patch for me to add without duplicating it. I’ll leave this issue with the existing fix rather than open a redundant PR. Let me know if you see a remaining warning path in another spdlog file.

[rachelwelchio]

I checked current v1.x vs the released v1.17.0 tag:

• v1.17.0 still forwards format_string_t in a way that can invoke the deprecated fmt::fstring::operator string_view() with fmt 12.2.
• v1.x tip already uses fmt.get() at the logger boundary (follow-ups from Fix deprecated copy constructor usage of fmt::format_string #3541 / Fix format_string propagation #3543), so this should be fixed in the next spdlog release without a further code change.

I am not opening a redundant PR on tip. Happy to help if anything still warns after the next release cut.