diagnostic: Check for unnecessary Core and Cask taps

- If the user doesn't have `HOMEBREW_DEVELOPER` or
  `HOMEBREW_NO_INSTALL_FROM_API` set but does have `homebrew/core` or
  `homebrew/cask` taps installed this can cause problems with installing
  outdated software.
- Hence, warn them in `brew doctor` if they have either of these taps
  installed, with instructions on how to remove them.

Library/Homebrew/diagnostic.rb

@@ -842,6 +842,32 @@ module Homebrew
         EOS
       end
 
+      def check_for_unnecessary_core_tap
+        return if ENV["HOMEBREW_DEVELOPER"]
+        return if ENV["HOMEBREW_NO_INSTALL_FROM_API"]
+        return unless CoreTap.instance.installed?
+
+        <<~EOS
+          You have an unnecessary local Core tap!
+          This can cause problems installing up-to-date formulae.
+          Please remove it by running:
+           brew untap #{CoreTap.instance.name}
+        EOS
+      end
+
+      def check_for_unnecessary_cask_tap
+        return if ENV["HOMEBREW_DEVELOPER"]
+        return if ENV["HOMEBREW_NO_INSTALL_FROM_API"]
+        return unless (cask_tap = Tap.fetch("homebrew", "cask")).installed?
+
+        <<~EOS
+          You have an unnecessary local Cask tap.
+          This can cause problems installing up-to-date casks.
+          Please remove it by running:
+            brew untap #{cask_tap.name}
+        EOS
+      end
+
       def check_cask_software_versions
         add_info "Homebrew Version", HOMEBREW_VERSION
         add_info "macOS", MacOS.full_version

Library/Homebrew/test/diagnostic_checks_spec.rb

@@ -114,4 +114,24 @@ describe Homebrew::Diagnostic::Checks do
     expect(checks.check_homebrew_prefix)
       .to match("Your Homebrew's prefix is not #{Homebrew::DEFAULT_PREFIX}")
   end
+
+  specify "#check_for_unnecessary_core_tap" do
+    ENV.delete("HOMEBREW_DEVELOPER")
+    ENV.delete("HOMEBREW_NO_INSTALL_FROM_API")
+
+    allow(CoreTap).to receive(:installed?).and_return(true)
+
+    expect(checks.check_for_unnecessary_core_tap).to match("You have an unnecessary local Core tap")
+  end
+
+  specify "#check_for_unnecessary_cask_tap" do
+    ENV.delete("HOMEBREW_DEVELOPER")
+    ENV.delete("HOMEBREW_NO_INSTALL_FROM_API")
+
+    cask_tap = Tap.new("homebrew", "cask")
+    allow(Tap).to receive(:fetch).with("homebrew", "cask").and_return(cask_tap)
+    allow(cask_tap).to receive(:installed?).and_return(true)
+
+    expect(checks.check_for_unnecessary_cask_tap).to match("unnecessary local Cask tap")
+  end
 end