Allow for homebrew/core to be untapped

Library/Homebrew/cmd/untap.rb

@@ -25,7 +25,7 @@ module Homebrew
     args = untap_args.parse
 
     args.named.to_installed_taps.each do |tap|
-      odie "Untapping #{tap} is not allowed" if tap.core_tap?
+      odie "Untapping #{tap} is not allowed" if tap.core_tap? && !ENV["HOMEBREW_UNTAP_HOMEBREW_CORE"]
 
       installed_tap_formulae = Formula.installed.select { |formula| formula.tap == tap }
       installed_tap_casks = Cask::Caskroom.casks.select { |cask| cask.tap == tap }

Library/Homebrew/cmd/update-report.rb

@@ -207,6 +207,7 @@ module Homebrew
 
   def install_core_tap_if_necessary
     return if ENV["HOMEBREW_UPDATE_TEST"]
+    return if ENV["HOMEBREW_UNTAP_HOMEBREW_CORE"]
 
     core_tap = CoreTap.instance
     return if core_tap.installed?

Library/Homebrew/dependency.rb

@@ -46,6 +46,15 @@ class Dependency
     formula
   end
 
+  def unavailable_core_formula?
+    to_formula
+    false
+  rescue CoreTapFormulaUnavailableError
+    true
+  rescue
+    false
+  end
+
   def installed?
     to_formula.latest_version_installed?
   end
@@ -130,6 +139,8 @@ class Dependency
 
     def action(dependent, dep, &block)
       catch(:action) do
+        prune if dep.unavailable_core_formula?
+
         if block
           yield dependent, dep
         elsif dep.optional? || dep.recommended?

Library/Homebrew/exceptions.rb

@@ -221,6 +221,13 @@ class TapFormulaUnavailableError < FormulaUnavailableError
   end
 end
 
+# Raised when a formula in a the core tap is unavailable.
+class CoreTapFormulaUnavailableError < TapFormulaUnavailableError
+  def initialize(name)
+    super CoreTap.instance, name
+  end
+end
+
 # Raised when a formula in a specific tap does not contain a formula class.
 class TapFormulaClassUnavailableError < TapFormulaUnavailableError
   include FormulaClassUnavailableErrorModule

Library/Homebrew/formulary.rb

@@ -361,6 +361,10 @@ module Formulary
     end
 
     def get_formula(*)
+      if !CoreTap.instance.installed? && ENV["HOMEBREW_UNTAP_HOMEBREW_CORE"]
+        raise CoreTapFormulaUnavailableError, name
+      end
+
       raise FormulaUnavailableError, name
     end
   end

Library/Homebrew/tap.rb

@@ -734,6 +734,7 @@ class CoreTap < Tap
 
   def self.ensure_installed!
     return if instance.installed?
+    return if ENV["HOMEBREW_UNTAP_HOMEBREW_CORE"]
 
     safe_system HOMEBREW_BREW_FILE, "tap", instance.name
   end
@@ -748,9 +749,11 @@ class CoreTap < Tap
   end
 
   # @private
-  sig { void }
+  sig { params(manual: T::Boolean).void }
-  def uninstall
+  def uninstall(manual: false)
-    raise "Tap#uninstall is not available for CoreTap"
+    raise "Tap#uninstall is not available for CoreTap" unless ENV["HOMEBREW_UNTAP_HOMEBREW_CORE"]
+
+    super
   end
 
   # @private