[Bug c++/124300] New: Accepts invalid constexpr on anonymous union declaration

[suyuchang at whu dot edu.cn via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124300

            Bug ID: 124300
           Summary: Accepts invalid constexpr on anonymous union
                    declaration
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: suyuchang at whu dot edu.cn
  Target Milestone: ---

The code applies constexpr to an anonymous union declaration. Clang rejects the
code. GCC accepts it. 

Code:
union { } constexpr;

Command:
clang++ test.cpp

<source>:1:11: error: union cannot be marked constexpr
    1 | union { } constexpr;
      |           ^
1 error generated.

See Compiler Explorer: https://godbolt.org/z/sfM8Tanc7

The test case was generated by a fuzzer.