This ensures that each metavariable defined within a template (a template
parameter) has, by the time that the template definition is ended, at least
one reference to each metavariable.
This has practical benefits---ensuring that you haven't forgotten to use a
metavariable; ensuring that you clean up when code is removed; and ensuring
that you didn't accidentally delete some reference that you didn't intend to
(at least in the case of _all_ references...)---but the rationale for it in
this commit is a bit different. More on that below.
This does introduce the negative effect of making it more difficult to
consume inputs without utilizing them, acting more like a relevant type
system (in terms of substructural type systems and with respect to
metavariables, at least). You can for now reference them in contexts that
would reasonably have no effect on the program or be optimized away, but in
the future it'd be nice to explicitly state "hey this isn't intended to be
used yet", especially if you're creating shells of templates, or trying to
maintain BC in a particular situation. But more on that in the future.
For now, the real reason for this change is because of how I intend for
template expansion to work: by walking the body. Rather than having to
check both the parameters of the template and then expand the body
separately, we can simply trust that each parameter is referenced. Then,
after rebinding metavariable references, we can perform the same check on
the expansion template to see if there were arguments provided that do not
correspond to parameters.
This also adds flexibility with parameters that are used
conditionally---we'll be able to have conditionally required parameters in
error reporting.
More information on this is coming, though; it'll be included in the docs of
the commit that introduces the changes.
DEV-13163
Mike Gerwitz
bdd98a5d92