failed to build 2025.10.16 release against fmt 12.1.0

[chenrui333]

👋 seeing regression build failure with 2025.10.16 release against fmt 12.1.0

  In file included from /private/tmp/vcpkg-20251030-63553-iv56vj/vcpkg-tool-2025-10-16/src/vcpkg/base/checks.cpp:1:
  In file included from /private/tmp/vcpkg-20251030-63553-iv56vj/vcpkg-tool-2025-10-16/include/vcpkg/base/checks.h:3:
  /private/tmp/vcpkg-20251030-63553-iv56vj/vcpkg-tool-2025-10-16/include/vcpkg/base/lineinfo.h:22:1: error: reference to 'formatter' is ambiguous
     22 | VCPKG_FORMAT_WITH_TO_STRING(vcpkg::LineInfo);
        | ^
  /private/tmp/vcpkg-20251030-63553-iv56vj/vcpkg-tool-2025-10-16/include/vcpkg/base/fwd/fmt.h:28:17: note: expanded from macro 'VCPKG_FORMAT_WITH_TO_STRING'
     28 |     struct fmt::formatter<Type, Char, void> : fmt::formatter<std::string, Char, void>                                  \
        |                 ^
  /private/tmp/vcpkg-20251030-63553-iv56vj/vcpkg-tool-2025-10-16/include/vcpkg/base/fwd/fmt.h:8:16: note: candidate found by name lookup is 'fmt::v11::formatter'
      8 |         struct formatter;
        |                ^
  /opt/homebrew/include/fmt/base.h:655:8: note: candidate found by name lookup is 'fmt::v12::formatter'
    655 | struct formatter {
        |        ^

relates to Homebrew/homebrew-core#251812

[gvanem]

I get the same error with fmt ver. 12.1 (from today's git master). But experimenting, I came up with this patch:

--- a/include/vcpkg/base/fwd/fmt.h 2024-07-31 07:52:15
+++ b/include/vcpkg/base/fwd/fmt.h 2025-11-25 07:06:22
@@ -1,8 +1,12 @@
 #pragma once

+#ifndef FMT_MAJOR_VERSION
+#define FMT_MAJOR_VERSION v11
+#endif
+
 namespace fmt
 {
-    inline namespace v11
+    inline namespace FMT_MAJOR_VERSION
     {
         template<typename T, typename Char, typename Enable>
         struct formatter;

and CFLAGS += -DFMT_MAJOR_VERSION=v12.

This compiles and links vcpkg.exe from today's git master fine.

But I have to say this fmtlib stuff is a package from Hell. And all the obfuscated stuff going on with a
CMake-build and conflicting C++ standards does not improve things.