[libcxx] [llvm] [libc++] Kickstart C++29 support (PR #215820)

[Aaron Ballman via llvm-commits]

================
@@ -79,12 +79,17 @@
     "-flax-vector-conversions=none",
 ]
 
-_allStandards = ["c++03", "c++11", "c++14", "c++17", "c++20", "c++23", "c++26"]
+_allStandards = ["c++03", "c++11", "c++14", "c++17", "c++20", "c++23", "c++26", "c++29"]
 
 
 def getStdFlag(cfg, std):
     if hasCompileFlag(cfg, "-std=" + std):
         return "-std=" + std
+    fallbacks = {
+        "c++29": "c++2d",
----------------
AaronBallman wrote:

> @AaronBallman could you elaborate on what those problems are?

I mentioned them in https://github.com/llvm/llvm-project/issues/184053#issuecomment-3984266777 but the basic problem is: features in the in-progress standard are experimental until the standard is ratified but users are trained to think a `-std` flag with a specific number means the features are stable enough to rely on and both committees have done significant design changes to in-progress features.


https://github.com/llvm/llvm-project/pull/215820