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, 18 insertions, 0 deletions

diff --git a/cmake/TorchExternalFixes.cmake b/cmake/TorchExternalFixes.cmake
new file mode 100644
index 000000000..9dd5bf9e3
--- /dev/null
+++ b/cmake/TorchExternalFixes.cmake
@@ -0,0 +1,18 @@
+# Included in the TorchExternal sub-build via CMAKE_PROJECT_INCLUDE (runs right
+# after Torch's project() call).
+#
+# Torch's pinned spdlog bundles an fmt whose consteval format-string checking
+# fails to compile under newer AppleClang (Xcode 16+):
+#   error: call to consteval function 'fmt::basic_format_string<...>' is not a
+#   constant expression
+# Defining FMT_CONSTEVAL to empty falls back to fmt's pre-C++20 constexpr
+# checking, which compiles cleanly. Scoped to AppleClang so other platforms and
+# compilers are untouched. add_compile_definitions (rather than CMAKE_CXX_FLAGS)
+# because Torch's CMakeLists overwrites the latter.
+# TODO: Temporary hack. Delete this file (plus the CMAKE_PROJECT_INCLUDE line
+# in CMakeLists.txt 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()