val decorateToStringValue = PrivateMethod[String]('decorateToStringValue)
 

The type parameter on PrivateMethod, in this case String, is the result type of the private method you wish to invoke. The symbol passed to the PrivateMethod.apply factory method, in this case 'decorateToStringValue, is the name of the private method to invoke. To test the private method, use the invokePrivate operator, like this:

 targetObject invokePrivate decorateToStringValue(1)
 

Here, targetObject is a variable or singleton object name referring to the object whose private method you want to test. You pass the arguments to the private method in the parentheses after the PrivateMethod object. The result type of an invokePrivate operation will be the type parameter of the PrivateMethod object, thus you need not cast the result to use it. In other words, after creating a PrivateMethod object, the syntax to invoke the private method looks like a regular method invocation, but with the dot (.) replaced by invokePrivate. The private method is invoked dynamically via reflection, so if you have a typo in the method name symbol, specify the wrong result type, or pass invalid parameters, the invokePrivate operation will compile, but throw an exception at runtime.

One limitation to be aware of is that you can't use PrivateMethodTester to test a private method declared in a trait, because the class the trait gets mixed into will not declare that private method. Only the class generated to hold method implementations for the trait will have that private method. If you want to test a private method declared in a trait, and that method does not use any state of that trait, you can move the private method to a companion object for the trait and test it using PrivateMethodTester that way. If the private trait method you want to test uses the trait's state, your best options are to test it indirectly via a non-private trait method that calls the private method, or make the private method package access and test it directly via regular static method invocations.