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

[Aaron Ballman via cfe-commits]

================
@@ -8721,15 +8771,63 @@ bool LValueExprEvaluator::VisitMemberExpr(const 
MemberExpr *E) {
   return LValueExprEvaluatorBaseTy::VisitMemberExpr(E);
 }
 
+bool LValueExprEvaluator::VisitExtVectorElementExpr(
+    const ExtVectorElementExpr *E) {
+  bool Success = true;
+
+  APValue Val;
+  if (!Evaluate(Val, Info, E->getBase())) {
+    if (!Info.noteFailure())
+      return false;
+    Success = false;
+  }
+
+  SmallVector<uint32_t, 4> Indices;
+  E->getEncodedElementAccess(Indices);
+  // FIXME: support accessing more than one element
+  if (Indices.size() > 1)
+    return false;
+
+  if (Success) {
+    Result.setFrom(Info.Ctx, Val);
+    const VectorType *VT = E->getBase()->getType()->castAs<VectorType>();
+    HandleLValueVectorElement(Info, E, Result, VT->getElementType(),
+                              VT->getNumElements(), Indices[0]);
+  }
+
+  return Success;
+}
+
 bool LValueExprEvaluator::VisitArraySubscriptExpr(const ArraySubscriptExpr *E) 
{
-  // FIXME: Deal with vectors as array subscript bases.
-  if (E->getBase()->getType()->isVectorType() ||
-      E->getBase()->getType()->isSveVLSBuiltinType())
+  if (E->getBase()->getType()->isSveVLSBuiltinType())
     return Error(E);
 
   APSInt Index;
   bool Success = true;
 
+  if (const VectorType *VT = E->getBase()->getType()->getAs<VectorType>()) {
----------------
AaronBallman wrote:

```suggestion
  if (const auto *VT = E->getBase()->getType()->getAs<VectorType>()) {
```

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