macOS: Suppress default '-arch arm64' with CMAKE_<LANG>_COMPILER_TARGET

Since commit b6c60f14b6 (macOS: Default to arm64 architecture on Apple
Silicon hosts, 2020-09-28, v3.19.0-rc1~63^2) we add `-arch arm64` by
default on Apple Silicon hosts if `CMAKE_OSX_ARCHITECTURES` is not set.
This is necessary to prevent the toolchain from selecting its own
default architecture based on that of the build tool (e.g., `x86_64`).

If `CMAKE_<LANG>_COMPILER_TARGET` is set, its `-target` flag tells the
compiler what architecture to use, so do not add `-arch arm64`.

Fixes: #24599

1 files changed, 6 insertions, 2 deletions

diff --git a/Source/cmGeneratorTarget.cxx b/Source/cmGeneratorTarget.cxx
index 0b376c1..8a14eaf 100644
--- a/Source/cmGeneratorTarget.cxx
+++ b/Source/cmGeneratorTarget.cxx
@@ -3451,7 +3451,6 @@ std::string cmGeneratorTarget::GetCompilePDBDirectory(
 std::vector<std::string> cmGeneratorTarget::GetAppleArchs(
   std::string const& config, cm::optional<std::string> lang) const
 {
-  static_cast<void>(lang);
   std::vector<std::string> archVec;
   if (!this->IsApple()) {
     return archVec;
@@ -3468,7 +3467,12 @@ std::vector<std::string> cmGeneratorTarget::GetAppleArchs(
   if (archs) {
     cmExpandList(*archs, archVec);
   }
-  if (archVec.empty()) {
+  if (archVec.empty() &&
+      // Fall back to a default architecture if no compiler target is set.
+      (!lang ||
+       this->Makefile
+         ->GetDefinition(cmStrCat("CMAKE_", *lang, "_COMPILER_TARGET"))
+         .IsEmpty())) {
     this->Makefile->GetDefExpandList("_CMAKE_APPLE_ARCHS_DEFAULT", archVec);
   }
   return archVec;