[clang] [clang][ExprConst] allow single element access of vector object to be constant expression (PR #72607)

[Aaron Ballman via cfe-commits]

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@@ -437,6 +442,16 @@ namespace {
       MostDerivedArraySize = 2;
       MostDerivedPathLength = Entries.size();
     }
+    void addVectorUnchecked(QualType EltTy, uint64_t Size, uint64_t Idx) {
+      Entries.push_back(PathEntry::ArrayIndex(Idx));
+
+      // This is technically a most-derived object, though in practice this
+      // is unlikely to matter.
+      MostDerivedType = EltTy;
+      MostDerivedIsArrayElement = true;
+      MostDerivedArraySize = Size;
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AaronBallman wrote:

Until we have evidence that `&V[0]` makes sense for vector types, I think we should disallow it. It's easier for us to introduce support once we have a use case than to have to maintain support should someone start relying on this without us intending it as a stable extension.

https://github.com/llvm/llvm-project/pull/72607