[clang] [Clang][CodeGen] Bail out on constexpr unknown values in ConstantEmitter (PR #127525)

[A. Jiang via cfe-commits]

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+// RUN: %clang_cc1 -triple %itanium_abi_triple -std=c++23 %s -emit-llvm -o - | FileCheck %s
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frederick-vs-ja wrote:

This should be tested in old modes to prevent potential regressions.

P2280R4 is a defect report (ideally, against C++11/14/17/20), although Clang only applies it in C++23 and later currently, see also #127166. I think Clang's choice is currently reasonable, but it should changed one day.

https://github.com/llvm/llvm-project/pull/127525