Diagnostic: require CLT headers on 10.14

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

@@ -20,6 +20,7 @@ module Homebrew
           check_xcode_minimum_version
           check_clt_minimum_version
           check_if_xcode_needs_clt_installed
+          check_if_clt_needs_headers_installed
         ].freeze
       end
 
@@ -135,6 +136,17 @@ module Homebrew
         EOS
       end
 
+      def check_if_clt_needs_headers_installed
+        return unless MacOS::CLT.separate_header_package?
+        return if MacOS::CLT.headers_installed?
+
+        <<~EOS
+          The Command Line Tools header package must be installed on #{MacOS.version.pretty_name}.
+          The installer is located at:
+            #{MacOS::CLT::HEADER_PKG_PATH.sub(":macos_version", MacOS.version)}
+        EOS
+      end
+
       def check_for_other_package_managers
         ponk = MacOS.macports_or_fink
         return if ponk.empty?

Library/Homebrew/test/os/mac/diagnostic_spec.rb

@@ -32,6 +32,19 @@ describe Homebrew::Diagnostic::Checks do
       .to match("Xcode alone is not sufficient on El Capitan")
   end
 
+  specify "#check_if_clt_needs_headers_installed" do
+    allow(MacOS).to receive(:version).and_return(OS::Mac::Version.new("10.14"))
+    allow(MacOS::CLT).to receive(:installed?).and_return(true)
+    allow(MacOS::CLT).to receive(:headers_installed?).and_return(false)
+
+    expect(subject.check_if_clt_needs_headers_installed)
+      .to match("The Command Line Tools header package must be installed on Mojave.")
+
+    allow(MacOS).to receive(:version).and_return(OS::Mac::Version.new("10.13"))
+    expect(subject.check_if_clt_needs_headers_installed)
+      .to be_nil
+  end
+
   specify "#check_homebrew_prefix" do
     # the integration tests are run in a special prefix
     expect(subject.check_homebrew_prefix)