Specifying default array values in an optional array parameter
It is not a "constant expression", an expression that can be entirely evaluated at compile time and produces a single simple value that can be stored in the assembly metadata. To be used, later, in other code that makes the call. Optional values are retrieved at compile time, not runtime, strictly a compiler feature.
The New operator must be executed at runtime and requires code to do so. And is therefore not a constant expression. The simple workaround is to use Nothing and put the code in the method body:
Sub Foo(Optional ByVal myArray As Long() = Nothing) If myArray Is Nothing Then myArray = New Long() {0, 1} '' etc...End Sub