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When instantiating a constrained hidden template friend, we substitute
into its template-head requirements in tsubst_friend_function. For this
substitution we use the template's full argument vector whose outer
levels correspond to the instantiated class's arguments and innermost
level corresponds to the template's own level-lowered generic arguments.
But for A<int>::f here, for which the relevant argument vector is
{{int}, {Us...}}, the substitution into (C<Ts, Us> && ...) triggers the
assert in use_pack_expansion_extra_args_p since one argument is a pack
expansion and the other isn't.
And for A<int, int>::f, for which the relevant argument vector is
{{int, int}, {Us...}}, the use_pack_expansion_extra_args_p assert would
also trigger but we first get a bogus "mismatched argument pack lengths"
error from tsubst_pack_expansion.
Sidestepping the question of whether tsubst_pack_expansion should be
able to handle such substitutions, it seems we can work around this by
using only the instantiated class's arguments and not also the template
friend's own generic arguments, which is consistent with how we normally
substitute into the signature of a member template.
PR c++/107853
gcc/cp/ChangeLog:
* constraint.cc (maybe_substitute_reqs_for): Substitute into
the template-head requirements of a template friend using only
its outer arguments via outer_template_args.
* cp-tree.h (outer_template_args): Declare.
* pt.cc (outer_template_args): Define, factored out and
generalized from ...
(ctor_deduction_guides_for): ... here.
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/concepts-friend12.C: New test.
* g++.dg/cpp2a/concepts-friend13.C: New test.
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