diff --git a/Library/Homebrew/extend/os/mac/diagnostic.rb b/Library/Homebrew/extend/os/mac/diagnostic.rb
index 5c603ca628..f0b923f211 100644
--- a/Library/Homebrew/extend/os/mac/diagnostic.rb
+++ b/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"
diff --git a/Library/Homebrew/extend/os/mac/extend/ENV/super.rb b/Library/Homebrew/extend/os/mac/extend/ENV/super.rb
index ee0d2343cd..287e838eaf 100644
--- a/Library/Homebrew/extend/os/mac/extend/ENV/super.rb
+++ b/Library/Homebrew/extend/os/mac/extend/ENV/super.rb
@@ -9,7 +9,7 @@ module Superenv
   end
 
   def effective_sysroot
-    MacOS::Xcode.without_clt? ? MacOS.sdk_path.to_s : nil
+    MacOS.sdk_path.to_s if MacOS::Xcode.without_clt?
   end
 
   def homebrew_extra_paths
@@ -91,10 +91,8 @@ module Superenv
     generic_setup_build_environment(formula)
     self["HOMEBREW_SDKROOT"] = effective_sysroot
 
-    if MacOS::Xcode.without_clt? || MacOS::Xcode.version.to_i >= 7
-      self["MACOSX_DEPLOYMENT_TARGET"] = MacOS.version.to_s
-      self["SDKROOT"] = MacOS.sdk_path
-    end
+    self["MACOSX_DEPLOYMENT_TARGET"] = MacOS.version.to_s
+    self["SDKROOT"] = MacOS.sdk_path if MacOS::Xcode.without_clt?
 
     # Filter out symbols known not to be defined since GNU Autotools can't
     # reliably figure this out with Xcode 8 and above.
diff --git a/Library/Homebrew/os/mac.rb b/Library/Homebrew/os/mac.rb
index 9215e3c96c..ade801c42a 100644
--- a/Library/Homebrew/os/mac.rb
+++ b/Library/Homebrew/os/mac.rb
@@ -31,16 +31,25 @@ module OS
       @version = nil
     end
 
-    def prerelease?
-      # TODO: bump version when new OS is released
-      version >= "10.14"
+    def latest_sdk_version
+      # TODO: bump version when new Xcode macOS SDK is released
+      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
diff --git a/Library/Homebrew/os/mac/xcode.rb b/Library/Homebrew/os/mac/xcode.rb
index 96be56e3b1..ae725d9482 100644
--- a/Library/Homebrew/os/mac/xcode.rb
+++ b/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