Partial specialization of formatter<string> after instantiation in gcc

[zptan]

Error

bazel-out/k8-opt/bin/external/fmt/libfmt/include/fmt/format.h:4076:1: error: partial specialization of 'struct fmt::v10::formatter<std::__cxx11::basic_string<_CharT>, Char>' after instantiation of 'struct fmt::v10::formatter<std::__cxx11::basic_string<char>, char, void>' [-fpermissive]
 4076 | FMT_FORMAT_AS(std::basic_string<Char>, basic_string_view<Char>);
      | ^~~~~~~~~~~~~
bazel-out/k8-opt/bin/external/fmt/libfmt/include/fmt/format.h:4078:1: error: partial specialization of 'struct fmt::v10::formatter<std::basic_string_view<Char>, Char>' after instantiation of 'struct fmt::v10::formatter<std::basic_string_view<char>, char, void>' [-fpermissive]
 4078 | FMT_FORMAT_AS(detail::std_string_view<Char>, basic_string_view<Char>);
      | ^~~~~~~~~~~~~

Env

OS: Ubuntu 22.04.3
Compiler: g++ 11.4.0

[vitaut]

Please provide a godbolt repro.

[zptan]

Tried with godbolt, but unable to repro, don't know if some other lib(s) caused this

I solved the issue by

• Replacing fmt::memory_buffer by std::string
• Removing #include <fmt/format.h>

[alex-700]

@vitaut

we faced the same problem too.

There is a minimal reproducer I was able to craft:
https://godbolt.org/z/dhjah7PMT

It compiles (e.g) in

• clang + c++20 + fmt/10.1.0
• clang + c++17 + fmt/10.1.0
• gcc + c++20 + fmt/10.0.0
• gcc + c++17 + fmt/10.1.0

Failes:

• gcc + c++20 + fmt/10.1.0

[vitaut]

Thanks for the repro, reopened the issue.

[pauldreik]

I git bisected this to commit 256a826

[vitaut]

Fixed in bbb784f. Thanks for reporting.

[windowsair]

Same issue. On older versions of fmtlib, moving #include <fmt/format.h> before #include <fmt/core.h> temporarily solves the problem.

[tocic]

@vitaut
Experiencing a similar issue with this test project. Interestingly, it works in 10.0.0 and trunk, but doesn't work in 10.1.1 and 10.2.1. If this is expected (i.e. fixed on trunk), could you please provide a new release?

[vitaut]

@tocic your example is incorrect: you should include fmt/format.h where formatter<string> is defined.

[tocic]

@vitaut You mean in lib.h? I thought that I can use fmt/core.h in headers and fmt/format.h in sources to reduce the compilation time. Does the second example from the docs applies only to std::string_view? Maybe I should use const char* instead?

Note that formatter<string_view>::format is defined in fmt/format.h so it has to be included in the source file.

[vitaut]

@tocic, the docs should be clearer about it but let's continue the discussion in #3942 since it's not directly related to the current issue.