Xcode: Explicitly specify default native architecture on macOS

When `CMAKE_OSX_ARCHITECTURES` is not specified, we add the Xcode
setting `ONLY_ACTIVE_ARCH = YES` with the intention of targeting the
native architecture of the host.  However, the default `ARCHS` value
chosen by "Xcode 12 Universal Apps" includes multiple architectures.
Add an explicit `ARCHS` setting with value `$(NATIVE_ARCH_ACTUAL)`
to tell Xcode to use the host's native architecture only.

Fixes: #20893

Help/release/3.18.rst

@@ -318,3 +318,17 @@ Other Changes
 
 * The :manual:`cmake-file-api(7)` "codemodel" version 2 "target" object gained
   a new ``precompileHeaders`` field in the ``compileGroups`` objects.
+
+Updates
+=======
+
+Changes made since CMake 3.18.0 include the following.
+
+3.18.1
+------
+
+* The :generator:`Xcode` generator, when :variable:`CMAKE_OSX_ARCHITECTURES`
+  is not defined, now selects ``$(NATIVE_ARCH_ACTUAL)`` as the default
+  architecture (the Xcode ``ARCHS`` setting).  This is needed for Xcode 12
+  to select the host's architecture, which older versions of Xcode did
+  by default.

Modules/CMakeDetermineCompilerId.cmake

@@ -469,6 +469,12 @@ Id flags: ${testflags} ${CMAKE_${lang}_COMPILER_ID_FLAGS_ALWAYS}
         set(id_clang_cxx_library "CLANG_CXX_LIBRARY = \"${CMAKE_MATCH_3}\";")
       endif()
     endif()
+    if(CMAKE_SYSTEM_NAME STREQUAL "Darwin" AND CMAKE_OSX_SYSROOT MATCHES "^$|[Mm][Aa][Cc][Oo][Ss]")
+      # When targeting macOS, use only the host architecture.
+      set(id_archs [[ARCHS = "$(NATIVE_ARCH_ACTUAL)";]])
+    else()
+      set(id_archs "")
+    endif()
     configure_file(${CMAKE_ROOT}/Modules/CompilerId/Xcode-3.pbxproj.in
       ${id_dir}/CompilerId${lang}.xcodeproj/project.pbxproj @ONLY)
     unset(_ENV_MACOSX_DEPLOYMENT_TARGET)

Modules/CompilerId/Xcode-3.pbxproj.in

@@ -84,6 +84,7 @@
 				CODE_SIGNING_REQUIRED = NO;
 				CONFIGURATION_BUILD_DIR = "$(BUILD_DIR)";
 				SYMROOT = .;
+				@id_archs@
 				@id_toolset@
 				@id_lang_version@
 				@id_clang_cxx_library@

Source/cmGlobalXCodeGenerator.cxx

@@ -12,6 +12,7 @@
 
 #include <cm/memory>
 #include <cmext/algorithm>
+#include <cmext/string_view>
 
 #include "cmsys/RegularExpression.hxx"
 
@@ -3118,6 +3119,14 @@ bool cmGlobalXCodeGenerator::CreateXCodeObjects(
   if (archs.empty()) {
     // Tell Xcode to use NATIVE_ARCH instead of ARCHS.
     buildSettings->AddAttribute("ONLY_ACTIVE_ARCH", this->CreateString("YES"));
+    // When targeting macOS, use only the host architecture.
+    if (this->SystemName == "Darwin"_s &&
+        (!sysroot || !*sysroot ||
+         cmSystemTools::LowerCase(sysroot).find("macos") !=
+           std::string::npos)) {
+      buildSettings->AddAttribute("ARCHS",
+                                  this->CreateString("$(NATIVE_ARCH_ACTUAL)"));
+    }
   } else {
     // Tell Xcode to use ARCHS (ONLY_ACTIVE_ARCH defaults to NO).
     buildSettings->AddAttribute("ARCHS", this->CreateString(archs));
@@ -3217,7 +3226,8 @@ void cmGlobalXCodeGenerator::ComputeArchitectures(cmMakefile* mf)
   }
 
   if (this->Architectures.empty()) {
-    // With no ARCHS we use ONLY_ACTIVE_ARCH.
+    // With no ARCHS we use ONLY_ACTIVE_ARCH and possibly a
+    // platform-specific default ARCHS placeholder value.
     // Look up the arch that Xcode chooses in this case.
     if (const char* arch = mf->GetDefinition("CMAKE_XCODE_ARCHS")) {
       this->ObjectDirArchDefault = arch;