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

[via cfe-commits]

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@@ -13587,10 +13587,22 @@ 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;
+    unsigned FixitDiagID = 0;
+    {
+      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) {
+      FixitDiagID = diag::note_typecheck_expression_not_modifiable_lvalue;
+      S.Diag(Loc, FixitDiagID) << E->getSourceRange() << Assign;
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Sirraide wrote:

The `FixitDiagID` can just be inlined here.

This is also missing the actual fix-it hint; I’d suggest looking for uses of `FixItHint::CreateInsertion` and working with that.

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