fail with warning when calling private messages

lib/rspec/matchers/built_in/be.rb

@@ -135,6 +135,12 @@ def initialize(*args, &block)
 
         def matches?(actual)
           @actual = actual
+
+          if actual.private_methods.include? predicate
+            @error_message = "but it's a private method"
+            return
+          end
+
           begin
             return @result = actual.__send__(predicate, *@args, &@block)
           rescue NameError => predicate_missing_error
@@ -149,11 +155,11 @@ def matches?(actual)
         end
 
         def failure_message_for_should
-          "expected #{predicate}#{args_to_s} to return true, got #{@result.inspect}"
+          "expected #{predicate}#{args_to_s} to return true, #{@error_message || "got #{@result.inspect}"}"
         end
 
         def failure_message_for_should_not
-          "expected #{predicate}#{args_to_s} to return false, got #{@result.inspect}"
+          "expected #{predicate}#{args_to_s} to return false, #{@error_message || "got #{@result.inspect}"}"
         end
 
         def description