fmtlib does not compile with MSVC v140

[csandmann]

Issue

The MSVC v140 build breaks with the attached errors.

format.cc
fmt\include\fmt\base.h(743): error C2327: 'fmt::v11::basic_specs::data_': is not a type name, static, or enumerator [fmt\build\fmt.vcxproj]
fmt\include\fmt\base.h(743): error C2065: 'data_': undeclared identifier [fmt\build\fmt.vcxproj]
fmt\include\fmt\base.h(743): error C2338:  [fmt\build\fmt.vcxproj]

System

Visual Studio 2022 (17.12.4) with MSVC v140 - VS 2015 C++-Buildtools (v14.00)

Details

• Latest visual studio toolchain (v1942) builds without issues

Steps to reproduce

1. git checkout v1.15.1
2. cmake -T v140 -S . -B build/ && cmake --build build/

[sergiud]

Even if one fixes this issue there's still a ton of different seemingly unrelated compile errors starting with the 11.1.x release series. Bisecting leads to 761d35f as the first bad commit.

[vitaut]

This looks like a bug in specific compiler version considering that data_ is defined in

fmt/include/fmt/base.h

Line 742 in d42a068

unsigned data_ = 1 << fill_size_shift;

and it works on other versions of the same compiler. I recommend updating to a newer compiler version, e.g. v141 which is known to work: https://github.com/fmtlib/fmt/actions/runs/13189537160/job/36820689152. If there is an easy workaround, a PR would be welcome too.

[edo9300]

A simple workaround could be to skip that static assert on msvc, since it will always be true

[vitaut]

I think it's fine to remove the static assert if it helps (a PR is welcome), it doesn't add much value anyway.

[sergiud]

No need to remove the static_assert. Qualifying the member is sufficient:

diff --git a/include/fmt/base.h b/include/fmt/base.h
index 8fdcea7c..ce71ad98 100644
--- a/include/fmt/base.h
+++ b/include/fmt/base.h
@@ -740,7 +740,7 @@ class basic_specs {
   };
 
   unsigned data_ = 1 << fill_size_shift;
-  static_assert(sizeof(data_) * CHAR_BIT >= 18, "");
+  static_assert(sizeof(basic_specs::data_) * CHAR_BIT >= 18, "");
 
   // Character (code unit) type is erased to prevent template bloat.
   char fill_data_[max_fill_size] = {' '};

As previously mentioned, however, this does not fix the compilation errors using the vc140 toolset because more errors arise after a fix.

[vitaut]

v140 is pretty old and we don't claim compatibility with it. But as I wrote earlier, if there is an easy fix, a PR would be welcome.