KegOnlyReason: Reasons related to macOS valid only on macOS

Library/Homebrew/extend/os/formula_support.rb

@@ -0,0 +1 @@
+require "extend/os/mac/formula_support" if OS.mac?

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

@@ -0,0 +1,5 @@
+class KegOnlyReason
+  def valid?
+    true
+  end
+end

Library/Homebrew/formula_support.rb

@@ -12,7 +12,7 @@ class KegOnlyReason
   end
 
   def valid?
-    true
+    ![:provided_by_macos, :provided_by_osx, :shadowed_by_macos].include?(@reason)
   end
 
   def to_s
@@ -61,3 +61,5 @@ class BottleDisableReason
     @reason
   end
 end
+
+require "extend/os/formula_support"