Currently it is the implicit function template code in pt.c that updates
this; specifically add_implicit_template_parms and
finish_fully_implicit_template.
It is queried by the parser (both in parser.c and lambda.c) to decide
whether a new [implicit] template parameter list as been started and to
decide how to finish up.
I had a look into this; it should be possible to move these out of pt.c
and into parser.c (or some new file generic-parms.c, implicit-pt.c or
some such) and possibly add a plain global counter for this state,
rather than attribute it to each scope.