@MergeMethod annotation should check that a replaced method actually exists

[Fifty-Nine]

The @MergeMethod annotation allows a user to augment an existing method on a class. However, no checks are done to verify that such a method actually exists. In most cases, this isn't a major issue since the user can use the @Override annotation to ensure such a method exists on the parent class.

However, there are cases where using @Override isn't actually possible. In particular, static methods can't use @Override since you can't use virtual dispatch with a static method. In such a case, it's possible to specify an incorrect signature for a @MergeMethod, and would look like a silent failure from a user perspective: the method wouldn't be overridden, and the default behavior for the overridden method would remain. In most cases this would just be a silent failure, but in some cases if there are multiple merged methods this could lead to crashes or other weird behavior.

All this would be mitigated by issuing an error or warning when a @MergeMethod does not replace something.

[benjholla]

Good catch. Adding a warning message in the Error Log for now until it can be integrated into the builder.

[benjholla]

Warning added in f657a27.