[clang] [clang] Fix-it hint for `++this` -> `++*this` when deref is modifiable (PR #94159)

[Rajveer Singh Bharadwaj via cfe-commits]

================
@@ -13587,10 +13602,47 @@ static bool CheckForModifiableLvalue(Expr *E, SourceLocation Loc, Sema &S) {
   SourceRange Assign;
   if (Loc != OrigLoc)
     Assign = SourceRange(OrigLoc, OrigLoc);
-  if (NeedType)
+  if (NeedType) {
     S.Diag(Loc, DiagID) << E->getType() << E->getSourceRange() << Assign;
-  else
+  } else {
+    ExprResult Deref;
+    {
+      Sema::TentativeAnalysisScope Trap(S);
+      Deref = S.ActOnUnaryOp(S.getCurScope(), Loc, tok::star, E);
+    }
     S.Diag(Loc, DiagID) << E->getSourceRange() << Assign;
+    if (Deref.isUsable() &&
+        Deref.get()->isModifiableLvalue(S.Context, &Loc) == Expr::MLV_Valid) {
+      std::string exprType;
+      switch (E->getStmtClass()) {
+      case Stmt::CStyleCastExprClass: {
+        const auto *CSC = cast<CStyleCastExpr>(E);
+        exprType = CSC->getType().getAsString();
+        break;
+      }
+      case Stmt::CXXConstCastExprClass: {
+        const auto *CXXCCE = cast<CXXConstCastExpr>(E);
+        exprType = CXXCCE->getTypeAsWritten().getAsString();
+        break;
+      }
+      case Stmt::CXXReinterpretCastExprClass: {
+        const auto *CXXRCE = cast<CXXReinterpretCastExpr>(E);
+        exprType = CXXRCE->getTypeAsWritten().getAsString();
+        break;
+      }
+      case Stmt::CXXThisExprClass: {
+        exprType = "this";
+        break;
+      }
+      default: {
+      }
+      }
+      S.Diag(Loc, diag::note_typecheck_expression_not_modifiable_lvalue)
+          << E->getSourceRange() << Assign
+          << FixItHint::CreateInsertion(E->getBeginLoc(),
+                                        "*(" + exprType + ")");
+    }
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Rajveer100 wrote:

Ahh, I thought of making it more descriptive (like Rust), you want me to remove this and just suggest `*(...)`?

https://github.com/llvm/llvm-project/pull/94159