C++ PATCH for c++/82331, ICE with variadic specialization of auto template parm

[Jason Merrill <jason at redhat dot com>]

The problem here is that deduction of F tries to substitute into the
type of F, but we haven't deduced R or A yet, so we can't do anything.
tsubst_pack_expansion knows how to handle this, but only expects to
see it within a template.  In general, substitution in the middle of
deduction needs to be treated as happening in template context.

Tested x86_64-pc-linux-gnu, applying to trunk.

Attachment: 82331.diff
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