llvm: also write *-apple-macosx* config files

Fixes #197278.

Also, remove the CLT requirement for pouring bottles. Many users use
this for the libraries, which don't need the CLT. We can add it back if
users report getting tripped up by this.

Co-authored-by: Bo Anderson <[email protected]>

Formula/l/llvm.rb

@@ -32,9 +32,6 @@ class Llvm < Formula
     sha256 cellar: :any_skip_relocation, x86_64_linux:  "479d06278bca8d5a7b8863f003ca127641ffef9c734976eee34fe8c0cc01d763"
   end
 
-  # Clang cannot find system headers if Xcode CLT is not installed
-  pour_bottle? only_if: :clt_installed
-
   keg_only :provided_by_macos
 
   # https://llvm.org/docs/GettingStarted.html#requirement
@@ -449,16 +446,11 @@ def install
       xctoolchain.parent.install_symlink xctoolchain.basename.to_s => "LLVM#{soversion}.xctoolchain"
 
       # Write config files for each macOS major version so that this works across OS upgrades.
-      # TODO: replace this with a call to `MacOSVersion.kernel_major_version` once this is in a release tag:
-      #   https://github.com/Homebrew/brew/pull/18674
-      {
-        11 => 20,
-        12 => 21,
-        13 => 22,
-        14 => 23,
-        15 => 24,
-      }.each do |macos_version, kernel_version|
-        write_config_files(macos_version, kernel_version, Hardware::CPU.arch)
+      MacOSVersion::SYMBOLS.each_key do |v|
+        macos_version = MacOSVersion.new(v)
+        next if macos_version < "10.14" # system headers are in `/usr` before this.
+
+        write_config_files(macos_version, MacOSVersion.kernel_major_version(macos_version), Hardware::CPU.arch)
       end
 
       # Also write an unversioned config file as fallback
@@ -504,19 +496,29 @@ def write_config_files(macos_version, kernel_version, arch)
     arches = Set.new([:arm64, :x86_64])
     arches << arch
 
+    systems = {
+      darwin: kernel_version,
+      macosx: macos_version,
+    }
     arches.each do |target_arch|
-      target_triple = "#{target_arch}-apple-darwin#{kernel_version}"
-      (clang_config_file_dir/"#{target_triple}.cfg").atomic_write <<~CONFIG
-        --sysroot=#{MacOS::CLT::PKG_PATH}/SDKs/MacOSX#{macos_version}.sdk
-      CONFIG
+      systems.each do |system, version|
+        config_file = "#{target_arch}-apple-#{system}#{version}.cfg"
+        (clang_config_file_dir/config_file).atomic_write <<~CONFIG
+          --sysroot=#{MacOS::CLT::PKG_PATH}/SDKs/MacOSX#{macos_version}.sdk
+        CONFIG
+      end
     end
   end
 
   def post_install
     return unless OS.mac?
-    return if (clang_config_file_dir/"#{Hardware::CPU.arch}-apple-darwin#{OS.kernel_version.major}.cfg").exist?
 
-    write_config_files(MacOS.version.major, OS.kernel_version.major, Hardware::CPU.arch)
+    config_files = %w[darwin macosx].map do |system|
+      clang_config_file_dir/"#{Hardware::CPU.arch}-apple-#{system}#{OS.kernel_version.major}.cfg"
+    end
+    return if config_files.all?(&:exist?)
+
+    write_config_files(MacOS.version, OS.kernel_version.major, Hardware::CPU.arch)
   end
 
   def caveats
@@ -534,6 +536,10 @@ def caveats
     on_macos do
       s += <<~EOS
 
+        Using `clang`, `clang++`, etc., requires a CLT installation at `/Library/Developer/CommandLineTools`.
+        If you don't want to install the CLT, you can write appropriate configuration files pointing to your
+        SDK at ~/.config/clang.
+
         To use the bundled libunwind please use the following LDFLAGS:
           LDFLAGS="-L#{opt_lib}/unwind -lunwind"
 
@@ -618,6 +624,16 @@ def caveats
         assert_equal "Hello World!", shell_output("./testCLT++").chomp
         system bin/"clang", "-v", "test.c", "-o", "testCLT"
         assert_equal "Hello World!", shell_output("./testCLT").chomp
+
+        target = "#{Hardware::CPU.arch}-apple-macosx#{MacOS.full_version}"
+        system bin/"clang-cpp", "-v", "--target=#{target}", "test.c"
+        system bin/"clang-cpp", "-v", "--target=#{target}", "test.cpp"
+
+        system bin/"clang", "-v", "--target=#{target}", "test.c", "-o", "test-macosx"
+        assert_equal "Hello World!", shell_output("./test-macosx").chomp
+
+        system bin/"clang++", "-v", "--target=#{target}", "-std=c++11", "test.cpp", "-o", "test++-macosx"
+        assert_equal "Hello World!", shell_output("./test++-macosx").chomp
       end
 
       # Testing Xcode