[Bug c++/86500] New: accepts-invalid with :: before decltype

[zhonghao at pku dot org.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86500

            Bug ID: 86500
           Summary: accepts-invalid with :: before decltype
           Product: gcc
           Version: 8.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zhonghao at pku dot org.cn
  Target Milestone: ---

g++ incorrectly accepts this:

struct S { struct T {}; };
::decltype(S())::T st;

clang++ rejects it:

error: expected unqualified-id