[clang] When checking for covariant return types, make sure the pointers or references are to *classes* (#111856)

https://eel.is/c++draft/class.virtual#8.1

This prevents overriding methods with non class return types that have
less cv-qualification.

Fixes: #111742

Author: bricknerb

clang/docs/ReleaseNotes.rst

@@ -99,6 +99,19 @@ C++ Specific Potentially Breaking Changes
     // Was error, now evaluates to false.
     constexpr bool b = f() == g();
 
+- Clang will now correctly not consider pointers to non classes for covariance.
+
+  .. code-block:: c++
+
+    struct A {
+      virtual const int *f() const;
+    };
+    struct B : A {
+      // Return type has less cv-qualification but doesn't point to a class.
+      // Error will be generated.
+      int *f() const override;
+    };
+
 - The warning ``-Wdeprecated-literal-operator`` is now on by default, as this is
   something that WG21 has shown interest in removing from the language. The
   result is that anyone who is compiling with ``-Werror`` should see this

clang/lib/Sema/SemaDeclCXX.cpp

@@ -18273,7 +18273,7 @@ bool Sema::CheckOverridingFunctionReturnType(const CXXMethodDecl *New,
   }
 
   // The return types aren't either both pointers or references to a class type.
-  if (NewClassTy.isNull()) {
+  if (NewClassTy.isNull() || !NewClassTy->isStructureOrClassType()) {
     Diag(New->getLocation(),
          diag::err_different_return_type_for_overriding_virtual_function)
         << New->getDeclName() << NewTy << OldTy

clang/test/SemaCXX/virtual-override.cpp

@@ -19,10 +19,12 @@ struct b { };
 
 class A {
   virtual a* f(); // expected-note{{overridden virtual function is here}}
+  virtual int *g(); // expected-note{{overridden virtual function is here}}
 };
 
 class B : A {
   virtual b* f(); // expected-error{{return type of virtual function 'f' is not covariant with the return type of the function it overrides ('b *' is not derived from 'a *')}}
+  virtual char *g(); // expected-error{{virtual function 'g' has a different return type ('char *') than the function it overrides (which has return type 'int *')}}
 };
 
 }
@@ -83,11 +85,15 @@ struct a { };
 class A {
   virtual const a* f(); 
   virtual a* g(); // expected-note{{overridden virtual function is here}}
+  virtual const int* h(); // expected-note{{overridden virtual function is here}}
+  virtual int* i(); // expected-note{{overridden virtual function is here}}
 };
 
 class B : A {
   virtual a* f(); 
   virtual const a* g(); // expected-error{{return type of virtual function 'g' is not covariant with the return type of the function it overrides (class type 'const a *' is more qualified than class type 'a *'}}
+  virtual int* h();  // expected-error{{virtual function 'h' has a different return type ('int *') than the function it overrides (which has return type 'const int *')}}
+  virtual const int* i(); // expected-error{{virtual function 'i' has a different return type ('const int *') than the function it overrides (which has return type 'int *')}}
 };
 
 }