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

[Richard Smith via cfe-commits]

================
@@ -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.
----------------
zygoloid wrote:

If LHS is `"abcdef" + 3` and RHS is `"cdef\0gh" + 1` then the overlapping region is `cdef\0` -- the portion of the two string literals that must overlap in memory if the pointers actually point to the same address at runtime.

Would it be helpful to add something like that to the comment?

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