Looks like this is a known bug in GCC: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72751

> On Jul 16, 2026, at 20:59, Russell Shaw via Std-Discussion <std-discussion_at_[hidden]> wrote:
>
> Hi,
>
> [class.union.anon] says "Nested types, anonymous unions, and functions
> shall not be declared within an anonymous union."
>
> It seems pretty definite. Everywhere i read and g++ allows them.
>
> struct A {
>
> union {
> union {
> int x;
> };
> };
>
> A() {
> int a = x;
> }
> };
>
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>