[Bug c++/87971] gcc allows nested namespace definition of inline namespaces

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87971

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It's also accepted without using nested namespace definitions, and that's
intentional.

namespace one {
  inline namespace two {}
}
namespace one {
  namespace two {}
}