This was (in part) GRECLIPSE-1538.

Consider the following:

package b; 

enum R { 
	A() { 
		@Override 
		String getFoo() { 
			
		} 
	}, 
	B() {
		 @Override 
		 String getFoo() { 
			 "bar" 
		 } 
	} 
	
	String getFoo() { 
		
	} 
} 

Now, select getFoo in either A or B definition and hit Ctrl+Shift+R to rename. Change it to getBar and hit enter: the following error dialog is shown:

Could not create a method handle for project 'null' with handle identifier '=MyGroovyProject/src/main/groovy<b{R.groovy[R[b.R$1~getFoo'

However, if you issue the same refactoring from the base getFoo() declaration, it works correctly.