The argument to MacOS.locate is always a string

Library/Homebrew/os/mac.rb

@@ -23,7 +23,7 @@ module OS
       # Don't call tools (cc, make, strip, etc.) directly!
       # Give the name of the binary you look for as a string to this method
       # in order to get the full path back as a Pathname.
-      (@locate ||= {}).fetch(tool.to_s) do |key|
+      (@locate ||= {}).fetch(tool) do |key|
         @locate[key] = if File.executable?(path = "/usr/bin/#{tool}")
           Pathname.new path
         # Homebrew GCCs most frequently; much faster to check this before xcrun