[Clang] Add fix-it hint to insert * when a modifiable lvalue is required (#94159)

This adds a fix-it hint in situations where a modifiable lvalue is expected, but a prvalue
or non-modifiable lvalue of pointer type was found, so long as dereferencing the pointer 
would result in a modifiable lvalue.

Resolves #93066

Author: Rajveer100

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -8779,6 +8779,9 @@ 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_add_deref_star_not_modifiable_lvalue : Note<
+  "add '*' to dereference it">; 
+
 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

@@ -13288,6 +13288,23 @@ 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(), "*");
+  }
+}
+
 /// Emit the "read-only variable not assignable" error and print notes to give
 /// more information about why the variable is not assignable, such as pointing
 /// to the declaration of a const variable, showing that a method is const, or
@@ -13382,6 +13399,7 @@ static void DiagnoseConstAssignment(Sema &S, const Expr *E,
         if (!DiagnosticEmitted) {
           S.Diag(Loc, diag::err_typecheck_assign_const)
               << ExprRange << ConstVariable << VD << VD->getType();
+          MaybeSuggestDerefFixIt(S, E, Loc);
           DiagnosticEmitted = true;
         }
         S.Diag(VD->getLocation(), diag::note_typecheck_assign_const)
@@ -13602,10 +13620,12 @@ 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 {
     S.Diag(Loc, DiagID) << E->getSourceRange() << Assign;
+    MaybeSuggestDerefFixIt(S, E, Loc);
+  }
   return true;
 }
 

clang/test/C/drs/dr1xx.c

@@ -296,6 +296,7 @@ void dr126(void) {
    */
   *object = 12; /* ok */
   ++object; /* expected-error {{cannot assign to variable 'object' with const-qualified type 'const IP' (aka 'int *const')}} */
+  /* expected-note@-1 {{add '*' to dereference it}} */
 }
 
 /* WG14 DR128: yes

clang/test/Sema/debug-93066.cpp

@@ -0,0 +1,49 @@
+// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify %s
+
+struct S {
+  void f() {
+    ++this; // expected-error {{expression is not assignable}}
+    // expected-note@-1 {{add '*' to dereference it}}
+  }
+
+  void g() const {
+    ++this; // expected-error {{expression is not assignable}}
+  }
+};
+
+void f(int* a, int* const b, const int* const c, __UINTPTR_TYPE__ d) {
+  // expected-note@-1 {{variable 'b' declared const here}}
+  // expected-note@-2 {{variable 'c' declared const here}}
+  (int*)d = 4; // expected-error {{assignment to cast is illegal, lvalue casts are not supported}}
+  // expected-note@-1 {{add '*' to dereference it}}
+    
+  ++a;
+  ++b; // expected-error {{cannot assign to variable 'b' with const-qualified type 'int *const'}}
+  // expected-note@-1 {{add '*' to dereference it}}
+  ++c; // expected-error {{cannot assign to variable 'c' with const-qualified type 'const int *const'}}
+
+  reinterpret_cast<int*>(42) += 3; // expected-error {{expression is not assignable}}
+  // expected-note@-1 {{add '*' to dereference it}}
+    
+  const int x = 42;
+  (const_cast<int*>(&x)) += 3; // expected-error {{expression is not assignable}}
+  // expected-note@-1 {{add '*' to dereference it}}
+}
+
+template <typename T>
+void f(T& t) {
+    // expected-note@* 2 {{variable 't' declared const here}}
+    ++t;
+    // expected-error@-1 {{cannot assign to variable 't' with const-qualified type 'int *const &'}}
+    // expected-error@-2 {{cannot assign to variable 't' with const-qualified type 'const int *const &'}}
+    // expected-note@-3 {{add '*' to dereference it}}
+}
+
+void g() {
+    int* a;
+    int* const b = a;
+    const int* const c = a;
+    f(a);
+    f(b); // expected-note {{in instantiation of function template specialization 'f<int *const>' requested here}}
+    f(c); // expected-note {{in instantiation of function template specialization 'f<const int *const>' requested here}}
+}

clang/test/Sema/exprs.c

@@ -65,8 +65,10 @@ void test4(void) {
 
 void test5(int *X, float *P) {
   (float*)X = P;   // expected-error {{assignment to cast is illegal, lvalue casts are not supported}}
+  // expected-note@-1 {{add '*' to dereference it}}
 #define FOO ((float*) X)
   FOO = P;   // expected-error {{assignment to cast is illegal, lvalue casts are not supported}}
+  // expected-note@-1 {{add '*' to dereference it}}
 }
 
 void test6(void) {

clang/test/Sema/va_arg_x86_32.c

@@ -2,5 +2,6 @@
 
 int a(void) {
   __builtin_va_arg((char*)0, int); // expected-error {{expression is not assignable}}
+  // expected-note@-1 {{add '*' to dereference it}}
   __builtin_va_arg((void*){0}, int); // expected-error {{first argument to 'va_arg' is of type 'void *'}}
 }

clang/test/SemaObjCXX/sel-address.mm

@@ -9,7 +9,20 @@ void h() {
   SEL s = @selector(dealloc);
   SEL* ps = &s;
 
+  /*
+   FIXME: https://github.com/llvm/llvm-project/pull/94159
+   
+   TLDR; This is about inserting '*' to deref.
+   
+   This would assign the value of s to the SEL object pointed to by
+   @selector(dealloc). However, in Objective-C, selectors are not pointers,
+   they are special compile-time constructs representing method names, and
+   they are immutable, so you cannot assign values to them.
+
+   Therefore, this syntax is not valid for selectors in Objective-C.
+   */
   @selector(dealloc) = s;  // expected-error {{expression is not assignable}}
+  // expected-note@-1 {{add '*' to dereference it}}
 
   SEL* ps2 = &@selector(dealloc);