Splicing dependent reflections of namespaces

[katzdm]

It is not yet possible to splice a dependent reflection of a namespace, e.g.,

template <std::meta::info R1, std::meta::info R2>
consteval void fn() {
  using namespace [:R1:];  // error
  namespace Alias = [:R2:];  // error
  // ...
}

//...

fn<^::, ^std>();

This represents the first time in the language that the name of a namespace can be dependent on a template parameter. My initial impression of this a while ago was that it wouldn't be a crazy amount of work, but probably a new Decl class will be needed to represent a dependent namespace, which will then be rebuilt during tree transform.

[katzdm]

Note that in the above example, using namespace [:R1:]; should probably be ill-formed for the same reason that using enum [:R1:] is ill-formed: If the namespace name is allowed to be dependent on a template parameter, then every subsequent unqualified lookup in the enclosing scope could possibly be a member of the namespace; it can't be known whether an identifier is undeclared until after template substitution.

So probably the only missing piece here is namespace Alias = [:R2:];, which probably requires a "dependent namespace" Decl class as described above.