KegOnlyReason: add reason helpers, rename valid.

Library/Homebrew/dev-cmd/audit.rb

@@ -396,8 +396,8 @@ module Homebrew
           end
 
           if @new_formula &&
-             dep_f.keg_only_reason&.reason == :provided_by_macos &&
-             dep_f.keg_only_reason.valid? &&
+             dep_f.keg_only_reason.provided_by_macos? &&
+             dep_f.keg_only_reason.applicable? &&
              !%w[apr apr-util openblas openssl openssl@1.1].include?(dep.name)
             new_formula_problem(
               "Dependency '#{dep.name}' is provided by macOS; " \
@@ -507,13 +507,14 @@ module Homebrew
       return unless @core_tap
 
       if formula.keg_only?
-        return if formula.keg_only_reason.reason == :versioned_formula
+        return if formula.keg_only_reason.versioned_formula?
         if formula.name.start_with?("openssl", "libressl") &&
-           formula.keg_only_reason.reason == :provided_by_macos
+           formula.keg_only_reason.provided_by_macos?
           return
         end
       end
 
+      # TODO: verify formulae still exist
       keg_only_whitelist = %w[
         autoconf@2.13
         bash-completion@2

Library/Homebrew/extend/os/mac/formula_support.rb

 # frozen_string_literal: true
 
 class KegOnlyReason
-  def valid?
+  def applicable?
     true
   end
 end

Library/Homebrew/formula.rb

@@ -1045,7 +1045,7 @@ class Formula
   def keg_only?
     return false unless keg_only_reason
 
-    keg_only_reason.valid?
+    keg_only_reason.applicable?
   end
 
   # @private

Library/Homebrew/formula_support.rb

@@ -13,24 +13,41 @@ class KegOnlyReason
     @explanation = explanation
   end
 
-  def valid?
-    ![:provided_by_macos, :provided_by_osx, :shadowed_by_macos].include?(@reason)
+  def versioned_formula?
+    @reason == :versioned_formula
+  end
+
+  def provided_by_macos?
+    @reason == :provided_by_macos
+  end
+
+  def shadowed_by_macos?
+    @reason == :shadowed_by_macos
+  end
+
+  def by_macos?
+    provided_by_macos? || shadowed_by_macos?
+  end
+
+  def applicable?
+    # macOS reasons aren't applicable on other OSs
+    # (see extend/os/mac/formula_support for override on macOS)
+    !by_macos?
   end
 
   def to_s
     return @explanation unless @explanation.empty?
 
-    case @reason
-    when :versioned_formula
+    if versioned_formula?
       <<~EOS
         this is an alternate version of another formula
       EOS
-    when :provided_by_macos
+    elsif provided_by_macos?
       <<~EOS
         macOS already provides this software and installing another version in
         parallel can cause all kinds of trouble
       EOS
-    when :shadowed_by_macos
+    elsif shadowed_by_macos?
       <<~EOS
         macOS provides similar software and installing this software in
         parallel can cause all kinds of trouble