Xcode: allow requesting Xcode/CLT SDKs specifically

Library/Homebrew/os/mac.rb

@@ -91,17 +91,7 @@ module OS
         CLTSDKLocator.new
       end
 
-      begin
-        sdk = if v.nil?
-          (Xcode.version.to_i >= 7) ? @locator.latest_sdk : @locator.sdk_for(version)
-        else
-          @locator.sdk_for v
-        end
-      rescue BaseSDKLocator::NoSDKError
-        sdk = @locator.latest_sdk
-      end
-      # Only return an SDK older than the OS version if it was specifically requested
-      sdk if v || (!sdk.nil? && sdk.version >= version)
+      @locator.sdk_if_applicable(v)
     end
 
     # Returns the path to an SDK or nil, following the rules set by #sdk.

Library/Homebrew/os/mac/sdk.rb

@@ -28,8 +28,26 @@ module OS
         SDK.new v, path
       end
 
+      def sdk_if_applicable(v = nil)
+        begin
+          sdk = if v.nil?
+            (source_version.to_i >= 7) ? latest_sdk : sdk_for(OS::Mac.version)
+          else
+            sdk_for v
+          end
+        rescue BaseSDKLocator::NoSDKError
+          sdk = latest_sdk
+        end
+        # Only return an SDK older than the OS version if it was specifically requested
+        sdk if v || (!sdk.nil? && sdk.version >= OS::Mac.version)
+      end
+
       private
 
+      def source_version
+        OS::Mac::Version::NULL
+      end
+
       def sdk_prefix
         ""
       end
@@ -56,6 +74,10 @@ module OS
     class XcodeSDKLocator < BaseSDKLocator
       private
 
+      def source_version
+        OS::Mac::Xcode.version
+      end
+
       def sdk_prefix
         @sdk_prefix ||= begin
           # Xcode.prefix is pretty smart, so let's look inside to find the sdk
@@ -73,6 +95,10 @@ module OS
     class CLTSDKLocator < BaseSDKLocator
       private
 
+      def source_version
+        OS::Mac::CLT.version
+      end
+
       # While CLT SDKs existed prior to Xcode 10, those packages also
       # installed a traditional Unix-style header layout and we prefer
       # using that

Library/Homebrew/os/mac/xcode.rb

@@ -96,6 +96,16 @@ module OS
         !prefix.nil?
       end
 
+      def sdk(v = nil)
+        @locator ||= XcodeSDKLocator.new
+
+        @locator.sdk_if_applicable(v)
+      end
+
+      def sdk_path(v = nil)
+        sdk(v)&.path
+      end
+
       def update_instructions
         if MacOS.version >= "10.9" && !OS::Mac.prerelease?
           <<~EOS
@@ -226,6 +236,16 @@ module OS
         end
       end
 
+      def sdk(v = nil)
+        @locator ||= CLTSDKLocator.new
+
+        @locator.sdk_if_applicable(v)
+      end
+
+      def sdk_path(v = nil)
+        sdk(v)&.path
+      end
+
       def update_instructions
         if MacOS.version >= "10.9"
           <<~EOS