[clang] [Clang] Reapply CWG2369 "Ordering between constraints and substitution" (PR #122423)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

This was discussed during Clang C/C++ Language Working Group meeting today, and the consensus seems that this PR shouldn't land before Clang 20 branch, which is scheduled to happen on January 28th.

https://github.com/llvm/llvm-project/pull/122423