[Bug c++/122391] New: [16 Regression] ICE at cp/tree.cc:6422: tree check: expected class 'type', have 'exceptional' (error_mark) in decl_linkage

[dhruvc at nvidia dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122391

            Bug ID: 122391
           Summary: [16 Regression] ICE at cp/tree.cc:6422: tree check:
                    expected class 'type', have 'exceptional' (error_mark)
                    in decl_linkage
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dhruvc at nvidia dot com
  Target Milestone: ---

Didn't triage down to the commit, this example was produced when trying to
reduce another ICE with creduce.

Code:

===
extern "C" {
  template <a> class b;
  struct {
    typedef b:
===

Link to compiler explorer: https://godbolt.org/z/x16Pa7PsK