Currently, Clang accepts the following code snippet since C++23 mode. Godbolt link.

constexpr int& r = r;

[dcl.ref]/6 (as clarified by the resolution of CWG453) indicates that such reference binding has UB, and thus needs to be rejected in constant evaluation.

Perhaps this is due to something missed in #95474.

Codegen change can be observed by dropping constexpr (Godbolt link), but this is conforming as such initialization raises UB.