[Clang] Regression on not rejecting UB in constexpr reference initialization due to implementing P2280R4

[frederick-vs-ja]

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.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: A. Jiang (frederick-vs-ja)

Currently, Clang accepts the following code snippet since C++23 mode. [Godbolt link](https://godbolt.org/z/5cxGrq6Gb).

constexpr int& r = r;

[[dcl.ref]/6](https://eel.is/c++draft/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.

[cor3ntin]

@shafik

[shafik]

This is obviously a bug