allow trailing comman in type arguments list

[Kingwl]

Fixes #21984

[Kingwl]

seems not my fault

[mhegazy]

@weswigham any objections to this change?

[bensaufley]

It seems like this, #21986, and #21984 are stagnating without comment/objection or merge/completion. What's the deal? If there's a reason not to allow this, and therefore not to merge this or #21986, and to close #21984, can it be posted somewhere?

[bensaufley]

Closed why?

[Kingwl]

@bensaufley my fault, i saw there are some related pr is existed...

[RyanCavanaugh]

I don't think we want to take this feature yet (issues marked Awaiting More Feedback generally shouldn't have PRs submitted). We're still trying to figure out what to do with partial type argument inference and this is just going to complicate the issue.