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

6 files changed, 33 insertions, 3 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;
diff --git a/Tests/RunCMake/AppleSilicon/RunCMakeTest.cmake b/Tests/RunCMake/AppleSilicon/RunCMakeTest.cmake
index 39e462e..58c50e0 100644
--- a/Tests/RunCMake/AppleSilicon/RunCMakeTest.cmake
+++ b/Tests/RunCMake/AppleSilicon/RunCMakeTest.cmake
@@ -10,11 +10,17 @@ function(run_arch case)
   run_cmake(${case})
   unset(RunCMake_TEST_OPTIONS)
   set(RunCMake_TEST_NO_CLEAN 1)
-  run_cmake_command(${case}-build ${CMAKE_COMMAND} --build . --config Debug)
+  set(RunCMake_TEST_OUTPUT_MERGE 1)
+  run_cmake_command(${case}-build ${CMAKE_COMMAND} --build . --config Debug --verbose)
 endfunction()
 
 run_arch(default)
 
+if(RunCMake_GENERATOR MATCHES "Makefiles|Ninja")
+  run_arch(default-target-arm64 -DCMAKE_C_COMPILER_TARGET=arm64-apple-macosx)
+  run_arch(default-target-x86_64 -DCMAKE_C_COMPILER_TARGET=x86_64-apple-macosx)
+endif()
+
 run_arch(arm64-var -DCMAKE_APPLE_SILICON_PROCESSOR=arm64)
 run_arch(x86_64-var -DCMAKE_APPLE_SILICON_PROCESSOR=x86_64)
 
diff --git a/Tests/RunCMake/AppleSilicon/default-target-arm64-build-check.cmake b/Tests/RunCMake/AppleSilicon/default-target-arm64-build-check.cmake
new file mode 100644
index 0000000..33ad931
--- /dev/null
+++ b/Tests/RunCMake/AppleSilicon/default-target-arm64-build-check.cmake
@@ -0,0 +1,5 @@
+if(NOT actual_stdout MATCHES "[ -]-target=arm64-apple-macosx ")
+  set(RunCMake_TEST_FAILED "No -target=arm64-apple-macosx flag found!")
+elseif(actual_stdout MATCHES " (-arch +[^ ]*)")
+  set(RunCMake_TEST_FAILED "'${CMAKE_MATCH_1}' flag incorrectly found!")
+endif()
diff --git a/Tests/RunCMake/AppleSilicon/default-target-arm64.cmake b/Tests/RunCMake/AppleSilicon/default-target-arm64.cmake
new file mode 100644
index 0000000..8c94020
--- /dev/null
+++ b/Tests/RunCMake/AppleSilicon/default-target-arm64.cmake
@@ -0,0 +1,5 @@
+enable_language(C)
+if(NOT CMAKE_OSX_ARCHITECTURES STREQUAL "")
+  message(FATAL_ERROR "CMAKE_OSX_ARCHITECTURES is '${CMAKE_OSX_ARCHITECTURES}', not empty ''")
+endif()
+add_library(arm64 arm64.c)
diff --git a/Tests/RunCMake/AppleSilicon/default-target-x86_64-build-check.cmake b/Tests/RunCMake/AppleSilicon/default-target-x86_64-build-check.cmake
new file mode 100644
index 0000000..9116ef4
--- /dev/null
+++ b/Tests/RunCMake/AppleSilicon/default-target-x86_64-build-check.cmake
@@ -0,0 +1,5 @@
+if(NOT actual_stdout MATCHES "[ -]-target=x86_64-apple-macosx ")
+  set(RunCMake_TEST_FAILED "No -target=x86_64-apple-macosx flag found!")
+elseif(actual_stdout MATCHES " (-arch +[^ ]*)")
+  set(RunCMake_TEST_FAILED "'${CMAKE_MATCH_1}' flag incorrectly found!")
+endif()
diff --git a/Tests/RunCMake/AppleSilicon/default-target-x86_64.cmake b/Tests/RunCMake/AppleSilicon/default-target-x86_64.cmake
new file mode 100644
index 0000000..ded46b7
--- /dev/null
+++ b/Tests/RunCMake/AppleSilicon/default-target-x86_64.cmake
@@ -0,0 +1,5 @@
+enable_language(C)
+if(NOT CMAKE_OSX_ARCHITECTURES STREQUAL "")
+  message(FATAL_ERROR "CMAKE_OSX_ARCHITECTURES is '${CMAKE_OSX_ARCHITECTURES}', not empty ''")
+endif()
+add_library(x86_64 x86_64.c)