In reply to evilpascal:
When have a simple set of items, including the empty set, you can always use an array concatenation {} to make an assignment any kind if unpacked array. There is no need to use an assignment pattern for anything you are writing. Use assignment patterns when you need replication, explicit types, or indexing of elements.
Use of an assignment pattern with an empty set of items is illegal according to SystemVerilog BNF. It seems that all tools allow this as an extension, but one tool is reporting it as a warning.