A using-declaration that expands a pack loses its ellipsis #1357

A using-declaration that expands a pack is documented without its .... For instance, the usual overload helper

template <class... Fs>
struct overloaded : Fs...
{
    using Fs::operator()...;
};

gets the synopsis using Fs::operator();, which isn't even valid C++: Fs is a pack and is never expanded.

Why it happens

UsingSymbol has no member saying whether the declaration is a pack expansion, so the generators have nothing to print the ellipsis from. Base classes already record it (IsPackExpansion, set from CXXBaseSpecifier::getEllipsisLoc in src/mrdocs/AST/ASTVisitor.cpp), and Clang gives the same information for a dependent using-declaration through isPackExpansion on UnresolvedUsingValueDecl and UnresolvedUsingTypenameDecl.

The golden fixture that #1317 adds for inheriting constructors shows the same thing for using Bases::Bases...;.