[Bug c++/90291] [8/9/10 Regression] Inline namespace erroneously extends another namespace

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90291

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Testcase without typos:

inline namespace A
{
  inline namespace B     // (1)
  {
    void foo() {}
  }
}

inline namespace B     // (2)
{
  inline namespace A
  {
    void foo() {}
  }
}