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

[via cfe-commits]

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@@ -13273,6 +13273,23 @@ enum {
   ConstUnknown,  // Keep as last element
 };
 
+static void MaybeSuggestDerefFixIt(Sema &S, const Expr *E, SourceLocation Loc) {
+  ExprResult Deref;
+  Expr *TE = const_cast<Expr *>(E);
+  {
+    Sema::TentativeAnalysisScope Trap(S);
+    Deref = S.ActOnUnaryOp(S.getCurScope(), Loc, tok::star, TE);
+  }
+  if (Deref.isUsable() &&
+      Deref.get()->isModifiableLvalue(S.Context, &Loc) == Expr::MLV_Valid &&
+      !E->getType()->isObjCObjectPointerType()) {
+    S.Diag(E->getBeginLoc(),
+           diag::note_typecheck_add_deref_star_not_modifiable_lvalue)
+        << E->getSourceRange()
+        << FixItHint::CreateInsertion(E->getBeginLoc(), "*");
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Sirraide wrote:

> I suspect we simply don't have enough information to do this from within CheckForModifiableLvalue.

Yeah, my idea was that by trying to build a `*` expression we might be able to figure out whether that makes sense in the context, whereas just looking for e.g. pointer types would cause too many false positives (at least that’s what I thought), but it might just be that you end up with too many false positives either way...

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