cask_loader: do not auto-tap in FromTapLoader

Library/Homebrew/cask/cask_loader.rb

@@ -157,7 +157,7 @@ module Cask
       end
 
       def load(config:)
-        tap.install unless tap.installed?
+        raise TapCaskUnavailableError.new(tap, token) unless tap.installed?
 
         super
       end

Library/Homebrew/cask/exceptions.rb

@@ -103,6 +103,27 @@ module Cask
     end
   end
 
+  # Error when a cask in a specific tap is not available.
+  #
+  # @api private
+  class TapCaskUnavailableError < CaskUnavailableError
+    extend T::Sig
+
+    attr_reader :tap
+
+    def initialize(tap, token)
+      super("#{tap}/#{token}")
+      @tap = tap
+    end
+
+    sig { returns(String) }
+    def to_s
+      s = super
+      s += "\nPlease tap it and then try again: brew tap #{tap}" unless tap.installed?
+      s
+    end
+  end
+
   # Error when a cask already exists.
   #
   # @api private

Library/Homebrew/cli/named_args.rb

@@ -132,6 +132,12 @@ module Homebrew
 
         raise unreadable_error if unreadable_error.present?
 
+        user, repo, short_name = name.downcase.split("/", 3)
+        if repo.present? && short_name.present?
+          tap = Tap.fetch(user, repo)
+          raise TapFormulaOrCaskUnavailableError.new(tap, short_name)
+        end
+
         raise FormulaOrCaskUnavailableError, name
       end
       private :load_formula_or_cask

Library/Homebrew/exceptions.rb

@@ -97,6 +97,25 @@ class FormulaOrCaskUnavailableError < RuntimeError
   end
 end
 
+# Raised when a formula or cask in a specific tap is not available.
+class TapFormulaOrCaskUnavailableError < FormulaOrCaskUnavailableError
+  extend T::Sig
+
+  attr_reader :tap
+
+  def initialize(tap, name)
+    super "#{tap}/#{name}"
+    @tap = tap
+  end
+
+  sig { returns(String) }
+  def to_s
+    s = super
+    s += "\nPlease tap it and then try again: brew tap #{tap}" unless tap.installed?
+    s
+  end
+end
+
 # Raised when a formula is not available.
 class FormulaUnavailableError < FormulaOrCaskUnavailableError
   extend T::Sig