[RFA] PATCH for PR c++/39639

[Jason Merrill]

Dodji Seketeli wrote:
> I thought about making cp_parser_template_argument spit an error when no
> argument were parsed and the next token was an ellipsis, but then I wasn't sure
> why the ellipsis detection and pack expansion weren't done in that function in
> the first place; those are done in cp_parser_template_argument_list. So I
> thought I should handle the error at the level of
> cp_parser_template_argument_list as well.

Well, since cp_parser_template_argument is only called by 
cp_parser_template_argument_list, it doesn't really matter
which one gives the error.  OK.

Jason