diagnostic: check if Xcode needs CLT installed.

Require the CLT on all but the latest version of macOS to avoid the
continuous workarounds for SDK weirdness.

Library/Homebrew/extend/os/mac/diagnostic.rb

@@ -19,6 +19,7 @@ module Homebrew
         %w[
           check_xcode_minimum_version
           check_clt_minimum_version
+          check_if_xcode_needs_clt_installed
         ].freeze
       end
 
@@ -124,6 +125,15 @@ module Homebrew
         EOS
       end
 
+      def check_if_xcode_needs_clt_installed
+        return unless MacOS::Xcode.needs_clt_installed?
+
+        <<~EOS
+          Xcode alone is not sufficient on #{MacOS.version.pretty_name}.
+          #{DevelopmentTools.installation_instructions}
+        EOS
+      end
+
       def check_for_osx_gcc_installer
         return unless MacOS.version < "10.7" || ((MacOS::Xcode.version || "0") > "4.1")
         return unless DevelopmentTools.clang_version == "2.1"

Library/Homebrew/os/mac.rb

@@ -31,16 +31,25 @@ module OS
       @version = nil
     end
 
-    def prerelease?
+    def latest_sdk_version
-      # TODO: bump version when new OS is released
+      # TODO: bump version when new Xcode macOS SDK is released
-      version >= "10.14"
+      Version.new "10.13"
+    end
+
+    def latest_stable_version
+      # TODO: bump version when new macOS is released
+      Version.new "10.13"
     end
 
     def outdated_release?
-      # TODO: bump version when new OS is released
+      # TODO: bump version when new macOS is released
       version < "10.11"
     end
 
+    def prerelease?
+      version > latest_stable_version
+    end
+
     def cat
       version.to_sym
     end

Library/Homebrew/os/mac/xcode.rb

@@ -40,6 +40,15 @@ module OS
         version < minimum_version
       end
 
+      def latest_sdk_version?
+        OS::Mac.version == OS::Mac.latest_sdk_version
+      end
+
+      def needs_clt_installed?
+        return false if latest_sdk_version?
+        without_clt?
+      end
+
       def outdated?
         return false unless installed?
         version < latest_version