diff --git a/Library/Homebrew/extend/os/formula_support.rb b/Library/Homebrew/extend/os/formula_support.rb
new file mode 100644
index 0000000000..e1708c93d9
--- /dev/null
+++ b/Library/Homebrew/extend/os/formula_support.rb
@@ -0,0 +1 @@
+require "extend/os/mac/formula_support" if OS.mac?
diff --git a/Library/Homebrew/extend/os/mac/formula_support.rb b/Library/Homebrew/extend/os/mac/formula_support.rb
new file mode 100644
index 0000000000..39309663d6
--- /dev/null
+++ b/Library/Homebrew/extend/os/mac/formula_support.rb
@@ -0,0 +1,5 @@
+class KegOnlyReason
+  def valid?
+    true
+  end
+end
diff --git a/Library/Homebrew/formula_support.rb b/Library/Homebrew/formula_support.rb
index 920db5839d..aef1a58ba5 100644
--- a/Library/Homebrew/formula_support.rb
+++ b/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"