[clang] Fix-it hint for ++this -> ++*this when deref is modifiable

Resolves #93066

Author: Rajveer100

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -8777,6 +8777,9 @@ def err_typecheck_incomplete_type_not_modifiable_lvalue : Error<
 def err_typecheck_lvalue_casts_not_supported : Error<
   "assignment to cast is illegal, lvalue casts are not supported">;
 
+def note_typecheck_expression_not_modifiable_lvalue : Note<
+  "dereference the pointer to modify">; 
+
 def err_typecheck_duplicate_vector_components_not_mlvalue : Error<
   "vector is not assignable (contains duplicate components)">;
 def err_block_decl_ref_not_modifiable_lvalue : Error<

clang/lib/Sema/SemaExpr.cpp

@@ -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;
+    }
+  }
   return true;
 }
 

clang/test/Sema/debug-93066.cpp

@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify %s
+
+struct S {
+  void f() {
+    ++this;
+    // expected-error@-1 {{expression is not assignable}}
+    // expected-note@-2 {{dereference the pointer to modify}}
+  }
+
+  void g() const {
+    ++this;
+    // expected-error@-1 {{expression is not assignable}}
+  }
+};