protobuf's hpb declares `class Arena { … backend::Arena arena_; }`, where
`namespace backend = hpb::internal::backend::upb;` and that namespace says
`using Arena = ::upb::Arena;`. cppTypeName dropped the leading `::`, so the
alias's target was looked up as `upb::Arena` from the alias's own scope,
whose nearest `upb` is that very namespace: the alias named itself, the
lookup ended on no class, and `arena_.Fuse(other.arena_)` fell back to the
type's last name, linking `hpb::Arena::Fuse` (and `IsFused`, `RefArena`) to
itself instead of `upb::Arena::Fuse`.
TypeName now records a leading `::` (`global`), and a global name is looked
up from the global scope alone: when an alias's target is followed (follow,
and the scope expand looks the target up from and hands on), in a class's
base clause (baseClassesOf), and in cppAliasedTypeWritten, which #2532's
`auto` locals and chains walk alias by alias.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Colby Mchenry