Avoid re-defining #formula?

Library/Homebrew/cli/args.rb

@@ -55,9 +55,15 @@ module Homebrew
       sig { returns(T::Boolean) }
       def build_from_source? = false
 
+      sig { returns(T::Boolean) }
+      def cask? = false
+
       sig { returns(T::Boolean) }
       def force_bottle? = false
 
+      # Defined in extend/os:
+      # def formula; end
+
       sig { returns(T::Boolean) }
       def HEAD? = false
 
@@ -122,9 +128,9 @@ module Homebrew
 
       sig { returns(T.nilable(Symbol)) }
       def only_formula_or_cask
-        if invoke_if_respond_to(:formula?) && !invoke_if_respond_to(:cask?)
+        if formula? && !cask?
           :formula
-        elsif invoke_if_respond_to(:cask?) && !invoke_if_respond_to(:formula?)
+        elsif cask? && !formula?
           :cask
         end
       end

Library/Homebrew/cli/parser.rb

@@ -393,10 +393,7 @@ module Homebrew
         end
 
         unless ignore_invalid_options
-          unless @is_dev_cmd
-            set_default_options
-            validate_options
-          end
+          validate_options unless @is_dev_cmd
           check_constraint_violations
           check_named_args(named_args)
         end
@@ -416,9 +413,6 @@ module Homebrew
         @args
       end
 
-      sig { void }
-      def set_default_options; end
-
       sig { void }
       def validate_options; end
 

Library/Homebrew/extend/os/linux/cli/parser.rb

@@ -9,15 +9,9 @@ module OS
 
         requires_ancestor { Homebrew::CLI::Parser }
 
-        sig { void }
-        def set_default_options
-          args.define_singleton_method(:formula?) { true } if args.respond_to?(:formula?)
-        end
-
         sig { void }
         def validate_options
-          return unless args.respond_to?(:cask?)
-          return unless T.unsafe(args).cask?
+          return unless args.cask?
 
           # NOTE: We don't raise an error here because we don't want
           #       to print the help page or a stack trace.

Library/Homebrew/extend/os/parser.rb

@@ -2,3 +2,12 @@
 # frozen_string_literal: true
 
 require "extend/os/linux/cli/parser" if OS.linux?
+
+module Homebrew
+  module CLI
+    class Args
+      sig { returns(T::Boolean) }
+      def formula? = OS.linux?
+    end
+  end
+end