[clang] [clang] implement current direction of CWG2765 for string literal comparisons in constant evaluation (PR #109208)

[Erich Keane via cfe-commits]

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@@ -2142,11 +2150,81 @@ static const ValueDecl *GetLValueBaseDecl(const LValue &LVal) {
   return LVal.Base.dyn_cast<const ValueDecl*>();
 }
 
-static bool IsLiteralLValue(const LValue &Value) {
-  if (Value.getLValueCallIndex())
+// Information about an LValueBase that is some kind of string.
+struct LValueBaseString {
+  std::string ObjCEncodeStorage;
+  StringRef Bytes;
+  int CharWidth;
+};
+
+// Gets the lvalue base of LVal as a string.
+static bool GetLValueBaseAsString(const EvalInfo &Info, const LValue &LVal,
+                                  LValueBaseString &AsString) {
+  const auto *BaseExpr = LVal.Base.dyn_cast<const Expr *>();
+  if (!BaseExpr)
+    return false;
+
+  // For ObjCEncodeExpr, we need to compute and store the string.
+  if (const auto *EE = dyn_cast<ObjCEncodeExpr>(BaseExpr)) {
+    Info.Ctx.getObjCEncodingForType(EE->getEncodedType(),
+                                    AsString.ObjCEncodeStorage);
+    AsString.Bytes = AsString.ObjCEncodeStorage;
+    AsString.CharWidth = 1;
+    return true;
+  }
+
+  // Otherwise, we have a StringLiteral.
+  const auto *Lit = dyn_cast<StringLiteral>(BaseExpr);
+  if (const auto *PE = dyn_cast<PredefinedExpr>(BaseExpr))
+    Lit = PE->getFunctionName();
+
+  if (!Lit)
+    return false;
+
+  AsString.Bytes = Lit->getBytes();
+  AsString.CharWidth = Lit->getCharByteWidth();
+  return true;
+}
+
+// Determine whether two string literals potentially overlap. This will be the
+// case if they agree on the values of all the bytes on the overlapping region
+// between them.
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erichkeane wrote:

I think that sort of example would be incredibly helpful next time I go through this.

https://github.com/llvm/llvm-project/pull/109208