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

["blitzrakete at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

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

            Bug ID: 87971
           Summary: gcc allows nested namespace definition of inline
                    namespaces
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: blitzrakete at gmail dot com
  Target Milestone: ---

gcc allows the following code with -std=c++17 without any warnings or errors
even with -Wall -Wextra -pedantic -pedantic-errors.

namespace one {
  inline namespace two {}
}

namespace one::two {} // ill-formed

two is an inline namespace, so it should be specified as such.