Warn when using Xcode 8 without CLT on 10.11

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

@@ -9,6 +9,7 @@ def development_tools_checks
           check_for_installed_developer_tools
           check_xcode_license_approved
           check_for_osx_gcc_installer
+          check_xcode_8_without_clt_on_el_capitan
         ]
       end
 
@@ -90,6 +91,19 @@ def check_clt_up_to_date
         EOS
       end
 
+      def check_xcode_8_without_clt_on_el_capitan
+        return unless MacOS::Xcode.without_clt?
+        # Scope this to Xcode 8 on El Cap for now
+        return unless MacOS.version == :el_capitan && MacOS::Xcode.version >= "8"
+
+        <<-EOS.undent
+          You have Xcode 8 installed without the CLT;
+          this causes certain builds to fail on OS X 10.11.
+          Please install the CLT via:
+            sudo xcode-select --install
+        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/test/test_os_mac_diagnostic.rb

@@ -38,4 +38,12 @@ def test_check_for_beta_xquartz
     MacOS::XQuartz.stubs(:version).returns("2.7.10_beta2")
     assert_match "The following beta release of XQuartz is installed: 2.7.10_beta2", @checks.check_for_beta_xquartz
   end
+
+  def test_check_xcode_8_without_clt_on_el_capitan
+    MacOS.stubs(:version).returns OS::Mac::Version.new("10.11")
+    MacOS::Xcode.stubs(:installed?).returns true
+    MacOS::Xcode.stubs(:version).returns "8.0"
+    MacOS::Xcode.stubs(:without_clt?).returns true
+    assert_match "You have Xcode 8 installed without the CLT", @checks.check_xcode_8_without_clt_on_el_capitan
+  end
 end