Bug 81196 Can't search for References, ... to an ITypeParameter [search]
org.eclipse.jdt.ui/ui/org/eclipse/jdt/ui/actions/FindImplementorsAction.java
public FindImplementorsAction(JavaEditor editor) {
		super(editor, SearchMessages.getString("Search.FindImplementorsAction.label"), new Class[] {ICompilationUnit.class, IType.class}); 
		init();
		PlatformUI.getWorkbench().getHelpSystem().setHelp(this, IJavaHelpContextIds.FIND_IMPLEMENTORS_IN_WORKSPACE_ACTION);
	}
	private void init() {
		setToolTipText(SearchMessages.getString("Search.FindImplementorsAction.tooltip")); 
		setImageDescriptor(JavaPluginImages.DESC_OBJS_SEARCH_DECL);
	}
	boolean canOperateOn(IJavaElement element) {
		if (!super.canOperateOn(element))
			return false;
		if (element.getElementType() == IJavaElement.TYPE)
			try {
				return ((IType) element).isInterface();
			} catch (JavaModelException ex) {
				ExceptionHandler.log(ex, SearchMessages.getString("Search.Error.javaElementAccess.message")); 
				return false;
			}
		
		return false;
	}
	int getLimitTo() {
		return IJavaSearchConstants.IMPLEMENTORS;
	}
	String getOperationUnavailableMessage() {
		return SearchMessages.getString("JavaElementAction.operationUnavailable.interface"); 
	}
}