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

[Nikolas Klauser 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",
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philnik777 wrote:

Can we file a bug against Clang that they should really support `-std=c++29`, so that we don't have to keep this garbage around?

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