Trailing comma in collection expressions

[ViIvanov]

It looks like the compiler supports trailing commas in collection expressions because the following code compiles successfully (SharpLab):

int[] list = [1, 2, 3, ];
// ----------------- ^ trailing comma

In the proposal Collection expressions, the syntax is described as:

+ collection_expression
  : '[' ']'
  | '[' collection_element ( ',' collection_element )* ']'
  ;

+ collection_element
  : expression_element
  | spread_element
  ;

+ expression_element
  : expression
  ;

+ spread_element
  : '..' expression
  ;

I understand it as "After the comma, the collection_element is required, that is, in the end, the expression."

Does this mean my understanding is incorrect or a different specification was used during development?

[CyrusNajmabadi]

A different specification was used during development. We'll fix up the lang spec. Thanks!