The inheriting constructor pack 'using T::T ...;' over a variadic base pack
wrapped the constructors of every base except the last, and emitted a spurious
Warning 526 naming the using declaration after the derived class instead of the
base.
During template instantiation the pack using declaration is expanded into one
using declaration per base: the first element is patched in place and the rest
are appended as siblings. The appended siblings were only revisited by the
class child walk when the using declaration was not the last child, because that
walk captures the next sibling before recursing so that an empty pack can detach
itself. As the pack using declaration is normally the last member, the appended
siblings kept the unexpanded template name and were never registered as
inheriting constructors of the instantiated class.
Expand and splice each appended sibling in place immediately after the first
element, so every base constructor is wrapped regardless of the using
declaration's position and the empty pack detach path is left untouched.
Closes#3481
Assisted-by: Claude Opus 4.8
William S Fulton