[clang] [analyzer] Check the correct first and last elements in cstring.UninitializedRead (PR #95408)

[Donát Nagy via cfe-commits]

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@@ -413,38 +588,18 @@ ProgramStateRef CStringChecker::CheckLocation(CheckerContext &C,
   if (!ER)
     return state;
 
-  SValBuilder &svalBuilder = C.getSValBuilder();
-  ASTContext &Ctx = svalBuilder.getContext();
-
   // Get the index of the accessed element.
-  NonLoc Idx = ER->getIndex();
-
-  if (CK == CharKind::Regular) {
-    if (ER->getValueType() != Ctx.CharTy)
-      return state;
-  } else {
-    if (ER->getValueType() != Ctx.WideCharTy)
-      return state;
-
-    QualType SizeTy = Ctx.getSizeType();
-    NonLoc WideSize =
-        svalBuilder
-            .makeIntVal(Ctx.getTypeSizeInChars(Ctx.WideCharTy).getQuantity(),
-                        SizeTy)
-            .castAs<NonLoc>();
-    SVal Offset = svalBuilder.evalBinOpNN(state, BO_Mul, Idx, WideSize, SizeTy);
-    if (Offset.isUnknown())
-      return state;
-    Idx = Offset.castAs<NonLoc>();
-  }
+  std::optional<NonLoc> Idx = getIndex(state, ER, CK);
+  if (!Idx)
+    return state;
 
   // Get the size of the array.
   const auto *superReg = cast<SubRegion>(ER->getSuperRegion());
   DefinedOrUnknownSVal Size =
       getDynamicExtent(state, superReg, C.getSValBuilder());
 
   ProgramStateRef StInBound, StOutBound;
-  std::tie(StInBound, StOutBound) = state->assumeInBoundDual(Idx, Size);
+  std::tie(StInBound, StOutBound) = state->assumeInBoundDual(*Idx, Size);
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NagyDonat wrote:

Perhaps apply a [structured binding](https://en.cppreference.com/w/cpp/language/structured_binding) instead of `std::tie`?

https://github.com/llvm/llvm-project/pull/95408