diff --git a/Library/Homebrew/extend/os/mac/diagnostic.rb b/Library/Homebrew/extend/os/mac/diagnostic.rb
index fe0bf0d5be..843196de9a 100644
--- a/Library/Homebrew/extend/os/mac/diagnostic.rb
+++ b/Library/Homebrew/extend/os/mac/diagnostic.rb
@@ -12,6 +12,7 @@ module Homebrew
           check_xcode_minimum_version
           check_clt_minimum_version
           check_if_xcode_needs_clt_installed
+          check_if_supported_sdk_available
         ].freeze
       end
 
@@ -357,6 +358,34 @@ module Homebrew
           Untap them with `brew untap`.
         EOS
       end
+
+      def check_if_supported_sdk_available
+        return unless MacOS.sdk_root_needed?
+        return if MacOS.sdk
+
+        locator = MacOS.sdk_locator
+
+        source = if locator.source == :clt
+          "CLT"
+        else
+          "Xcode"
+        end
+
+        all_sdks = locator.all_sdks
+        sdks_found_msg = unless all_sdks.empty?
+          <<~EOS
+            Homebrew found the following SDKs in the #{source} install:
+              #{locator.all_sdks.map(&:version).join("\n  ")}
+          EOS
+        end
+
+        <<~EOS
+          Could not find an SDK that supports macOS #{MacOS.version}.
+          You may have have an outdated or incompatible #{source}.
+          #{sdks_found_msg}
+          Please update #{source} or uninstall it if no updates are available.
+        EOS
+      end
     end
   end
 end
diff --git a/Library/Homebrew/os/mac.rb b/Library/Homebrew/os/mac.rb
index 27749cf0c8..fe658e924d 100644
--- a/Library/Homebrew/os/mac.rb
+++ b/Library/Homebrew/os/mac.rb
@@ -97,14 +97,16 @@ module OS
     # If no specific SDK is requested, the SDK matching the OS version is returned,
     # if available. Otherwise, the latest SDK is returned.
 
-    def sdk(v = nil)
-      @locator ||= if CLT.installed? && CLT.provides_sdk?
-        CLTSDKLocator.new
+    def sdk_locator
+      if CLT.installed? && CLT.provides_sdk?
+        CLT.sdk_locator
       else
-        XcodeSDKLocator.new
+        Xcode.sdk_locator
       end
+    end
 
-      @locator.sdk_if_applicable(v)
+    def sdk(v = nil)
+      sdk_locator.sdk_if_applicable(v)
     end
 
     def sdk_for_formula(f, v = nil)
diff --git a/Library/Homebrew/os/mac/sdk.rb b/Library/Homebrew/os/mac/sdk.rb
index 156c41812d..f6061c3712 100644
--- a/Library/Homebrew/os/mac/sdk.rb
+++ b/Library/Homebrew/os/mac/sdk.rb
@@ -31,6 +31,10 @@ module OS
         SDK.new v, path, source
       end
 
+      def all_sdks
+        sdk_paths.map { |v, p| SDK.new v, p, source }
+      end
+
       def sdk_if_applicable(v = nil)
         sdk = begin
           if v.nil?
@@ -47,15 +51,11 @@ module OS
         sdk
       end
 
-      private
-
       def source
         nil
       end
 
-      def source_version
-        OS::Mac::Version::NULL
-      end
+      private
 
       def sdk_prefix
         ""
@@ -81,15 +81,11 @@ module OS
     end
 
     class XcodeSDKLocator < BaseSDKLocator
-      private
-
       def source
         :xcode
       end
 
-      def source_version
-        OS::Mac::Xcode.version
-      end
+      private
 
       def sdk_prefix
         @sdk_prefix ||= begin
@@ -105,15 +101,11 @@ module OS
     end
 
     class CLTSDKLocator < BaseSDKLocator
-      private
-
       def source
         :clt
       end
 
-      def source_version
-        OS::Mac::CLT.version
-      end
+      private
 
       # While CLT SDKs existed prior to Xcode 10, those packages also
       # installed a traditional Unix-style header layout and we prefer
diff --git a/Library/Homebrew/os/mac/xcode.rb b/Library/Homebrew/os/mac/xcode.rb
index 1e852b412c..6190e267f4 100644
--- a/Library/Homebrew/os/mac/xcode.rb
+++ b/Library/Homebrew/os/mac/xcode.rb
@@ -107,10 +107,12 @@ module OS
         !prefix.nil?
       end
 
-      def sdk(v = nil)
-        @locator ||= XcodeSDKLocator.new
+      def sdk_locator
+        @sdk_locator ||= XcodeSDKLocator.new
+      end
 
-        @locator.sdk_if_applicable(v)
+      def sdk(v = nil)
+        sdk_locator.sdk_if_applicable(v)
       end
 
       def sdk_path(v = nil)
@@ -219,10 +221,12 @@ module OS
         version >= "8"
       end
 
-      def sdk(v = nil)
-        @locator ||= CLTSDKLocator.new
+      def sdk_locator
+        @sdk_locator ||= CLTSDKLocator.new
+      end
 
-        @locator.sdk_if_applicable(v)
+      def sdk(v = nil)
+        sdk_locator.sdk_if_applicable(v)
       end
 
       def sdk_path(v = nil)