Fix macOS arm64 build: fmt consteval error in the TorchExternal sub-build (#712)

* Fix macOS arm64 build: fmt consteval error in the TorchExternal sub-build

Torch's pinned spdlog bundles an fmt whose consteval format-string
checking fails to compile under newer AppleClang (Xcode 16+), breaking
the build-macos-arm64 CI job (the intel runner's older Xcode doesn't hit
it):

  error: call to consteval function 'fmt::basic_format_string<...>' is
  not a constant expression

Define FMT_CONSTEVAL as empty inside the Torch sub-build, falling back to
fmt's pre-C++20 constexpr checking. Injected via CMAKE_PROJECT_INCLUDE
because Torch's CMakeLists overwrites CMAKE_CXX_FLAGS, so plain flag
injection through CMAKE_ARGS is discarded. Scoped to AppleClang; no other
platform or compiler is affected, and the Torch pin is unchanged.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Also neutralize FMT_CONSTEVAL for the game build (vcpkg spdlog/fmt)

With the TorchExternal fix in place, CI reaches the game build and hits
the same consteval error again, this time from the vcpkg-provided
spdlog/fmt headers (SPDLOG_FMT_EXTERNAL) under the runner's Xcode 26.5.
Apply the same AppleClang-scoped FMT_CONSTEVAL= define at the project
level so both layers compile.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Pin vcpkg fmt to 10.2.1 (+ spdlog 1.14.1): fmt 11.0.2 can't compile on new AppleClang

The baseline's fmt 11.0.2 both fails to compile under newer AppleClang
(consteval format-string checking) and removed the #ifndef guard around
FMT_CONSTEVAL, so the AppleClang-scoped define from the previous commit
lands on the compile line but gets clobbered by fmt's own definition
(FMT_USE_CONSTEVAL is likewise unguarded in that version). No newer fmt
exists in the pinned baseline.

Override fmt to 10.2.1, which keeps the #ifndef guard, making the
existing define effective; spdlog moves to the matching 1.14.1 (1.15.x
requires fmt >= 11 headers).

Verified locally against the exact CI setup (vcpkg at baseline
2e58bb35ff, vcpkg toolchain file, Ninja): the previous failure reproduces
without this change and the full game builds cleanly with it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Add TODO markers for removing the fmt consteval workarounds

Requested in PR #712 review: mark all three workaround sites (project-level
define, Torch sub-build include, vcpkg fmt/spdlog pins) so they can be found
and removed together once the Torch pin is bumped past its spdlog/fmt update.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: siliconports <aguthmann10@gmail.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

1 files changed, 28 insertions, 0 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 45dff319a..53ae41d78 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -4,6 +4,23 @@ cmake_minimum_required(VERSION 3.16.0 FATAL_ERROR)
 project(Spaghettify VERSION 1.0.0 LANGUAGES C CXX ASM)
 include(FetchContent)
 
+# fmt's consteval format-string checking fails to compile under newer
+# AppleClang (Xcode 16+) with the spdlog/fmt versions in use (both vcpkg's on
+# CI and Torch's bundled copy — see cmake/TorchExternalFixes.cmake for the
+# latter):
+#   error: call to consteval function 'fmt::basic_format_string<...>' is not
+#   a constant expression
+# Defining FMT_CONSTEVAL as empty falls back to fmt's pre-C++20 constexpr
+# checking, which compiles cleanly with identical runtime behavior. Scoped to
+# AppleClang; other platforms and compilers are untouched.
+# TODO: Temporary hack. Remove this (plus cmake/TorchExternalFixes.cmake and
+# the vcpkg.json fmt/spdlog pins) once the Torch pin is bumped past Torch's
+# spdlog/fmt update. Note the bump also needs matching MK64 loader changes,
+# see the discussion on PR #712.
+if(APPLE AND CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+    add_compile_definitions("FMT_CONSTEVAL=")
+endif()
+
 set(NATO_PHONETIC_ALPHABET
   "Alfa" "Bravo" "Charlie" "Delta" "Echo" "Foxtrot" "Golf" "Hotel"
   "India" "Juliett" "Kilo" "Lima" "Mike" "November" "Oscar" "Papa"
@@ -704,10 +721,21 @@ add_custom_command(
 
 if(NOT CMAKE_SYSTEM_NAME STREQUAL "NintendoSwitch")
   include(ExternalProject)
+  # Torch's pinned spdlog bundles an fmt that trips a consteval error under
+  # newer AppleClang (Xcode 16+): "call to consteval function
+  # 'fmt::basic_format_string<...>' is not a constant expression". Neutralize
+  # FMT_CONSTEVAL inside the Torch sub-build via CMAKE_PROJECT_INCLUDE (Torch's
+  # CMakeLists overwrites CMAKE_CXX_FLAGS, so flags can't be injected that
+  # way). Fixes the build-macos-arm64 job; the intel runner's older Xcode
+  # doesn't hit it.
+  # TODO: Temporary hack. Remove the CMAKE_PROJECT_INCLUDE line below (plus
+  # cmake/TorchExternalFixes.cmake itself) once the Torch pin is bumped past
+  # Torch's spdlog/fmt update, see the discussion on PR #712.
   ExternalProject_Add(TorchExternal
     PREFIX TorchExternal
     SOURCE_DIR ${CMAKE_SOURCE_DIR}/torch
     CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=${CMAKE_BINARY_DIR}/torch
+               -DCMAKE_PROJECT_INCLUDE=${CMAKE_SOURCE_DIR}/cmake/TorchExternalFixes.cmake
 )
 
   ExternalProject_Get_Property(TorchExternal install_dir)