What is the ValueTask equivalent of Task.CompletedTask?

All structs have a default constructor. The default constructor of ValueTask creates a completed ValueTask:

var completedValueTask = new ValueTask();

Or alternatively:

ValueTask completedValueTask = default;

Update: The official documentation has been updated with the following note:

An instance created with the parameterless constructor or by the default(ValueTask) syntax (a zero-initialized structure) represents a synchronously, successfully completed operation.