[css-grid] Line names before auto-repeat

Hi,

with the current syntax [1]:
  <auto-track-list> =
    [ <line-names>? [ <fixed-size> | <fixed-repeat> ] ]+ <auto-repeat>
    [ <line-names>? [ <fixed-size> | <fixed-repeat> ] ]+ <line-names>?

The following declaration would be invalid:
  grid-template-columns: [first] repeat(auto-fill, 100px);

We don't see any good reason why this is invalid, actually this is right
now working in all the auto-repeat implementations (Firefox, Blink and
WebKit).

Are we missing something or it's just an oversight?

Thanks,
  Rego

[1] https://drafts.csswg.org/css-grid/#typedef-auto-track-list