c++: improve -Waddress warnings with *_cast [PR105569]

This patch improves the diagnostic for -Waddress when it warns for

  if (dynamic_cast<A*>(&ref))
    // ...

where 'ref' is a reference, which cannot be null.  In particular, it
changes
warning: comparing the result of pointer addition '(((A*)ref) + ((sizetype)(*(long int*)((& ref)->B::_vptr.B + -24))))' and NULL
to
warning: the compiler can assume that the address of 'ref' will never be NULL

	PR c++/105569

gcc/cp/ChangeLog:

	* typeck.cc (warn_for_null_address): Improve the warning when
	the POINTER_PLUS_EXPR's base is of reference type.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Waddress-9.C: New test.

gcc/cp/typeck.cc

@@ -4757,8 +4757,16 @@ warn_for_null_address (location_t location, tree op, tsubst_flags_t complain)
       tree off = TREE_OPERAND (cop, 1);
       if (!integer_zerop (off)
 	  && !warning_suppressed_p (cop, OPT_Waddress))
-	warning_at (location, OPT_Waddress, "comparing the result of pointer "
+	{
-		    "addition %qE and NULL", cop);
+	  tree base = TREE_OPERAND (cop, 0);
+	  STRIP_NOPS (base);
+	  if (TYPE_REF_P (TREE_TYPE (base)))
+	    warning_at (location, OPT_Waddress, "the compiler can assume that "
+			"the address of %qE will never be NULL", base);
+	  else
+	    warning_at (location, OPT_Waddress, "comparing the result of "
+			"pointer addition %qE and NULL", cop);
+	}
       return;
     }
   else if (CONVERT_EXPR_P (op)

gcc/testsuite/g++.dg/warn/Waddress-9.C

@@ -0,0 +1,31 @@
+// PR c++/105569
+// { dg-do compile { target c++11 } }
+// { dg-options -Waddress }
+
+class A {};
+
+class B : public virtual A {};
+
+class C : public A {};
+
+int main() {
+    B* object = new B();
+    B &ref = *object;
+
+    bool b = nullptr == dynamic_cast<A*>(&ref); // { dg-warning "the address of 'ref' will never be NULL" }
+    bool b4 = nullptr == static_cast<A*>(&ref); // { dg-warning "the address of 'ref' will never be NULL" }
+    if (dynamic_cast<A*>(&ref)) // { dg-warning "the address of 'ref' will never be NULL" }
+      {
+      }
+    if (static_cast<A*>(&ref)) // { dg-warning "the address of 'ref' will never be NULL" }
+      {
+      }
+
+    auto ptr = dynamic_cast<A*>(&ref);
+    bool b2 = ptr == nullptr;
+
+    C* cobject = new C();
+    C &cref = *cobject;
+
+    bool b3 = nullptr == dynamic_cast<A*>(&cref);
+}