[css-grid] Automatic position and span for named grid line

Hi,

reviewing the spec I've found the following sentence in
http://dev.w3.org/csswg/css-grid/#grid-placement-errors:
"If the grid item has an automatic position and a grid span for a named
line in a given dimension, instead treat the grid span as one."

If I understood it correctly, that means that something like this:
  grid-row: auto;
  grid-column: span namedgridline;

Will become:
  grid-row: auto;
  grid-column: span 1;

I'd like to confirm that this is the expected behaviour and the
rationale behind it.

BTW, current implementation in Blink allows to use a named grid line in
that case.

Thanks,
  Rego