[PATCH] Partially fix PR c++/12277 (Warn on dynamic cast with known NULL results)

[Patrick Palka]

On Tue, 2 Feb 2016, Jason Merrill wrote:

> On 11/09/2015 04:30 AM, Patrick Palka wrote:
>> +	      if (complain & tf_warning)
>> +		{
>> +		  if (VAR_P (old_expr))
>> +	            warning (0, "dynamic_cast of %q#D to %q#T can never 
>> succeed",
>> +				old_expr, type);
>> +		  else
>> +	            warning (0, "dynamic_cast of %q#E to %q#T can never 
>> succeed",
>> +				old_expr, type);
>> +		}
>> +	      return build_zero_cst (type);
>
> You also need to handle throwing bad_cast in the reference case.

Oops, fixed.  I also updated the test case to confirm that the expected
number of calls to bad_cast is generated.

-- >8 --

gcc/cp/ChangeLog:

 	PR c++/12277
 	* rtti.c (build_dynamic_cast_1): Warn on dynamic_cast that can
 	never succeed due to either the target type or the static type
 	being marked final.

gcc/testsuite/ChangeLog:

 	PR c++/12277
 	* g++.dg/rtti/dyncast8.C: New test.
---
  gcc/cp/rtti.c                        | 26 ++++++++++++++++++
  gcc/testsuite/g++.dg/rtti/dyncast8.C | 53 ++++++++++++++++++++++++++++++++++++
  2 files changed, 79 insertions(+)
  create mode 100644 gcc/testsuite/g++.dg/rtti/dyncast8.C

diff --git a/gcc/cp/rtti.c b/gcc/cp/rtti.c
index a43ff85..b1454d9 100644
--- a/gcc/cp/rtti.c
+++ b/gcc/cp/rtti.c
@@ -694,6 +694,32 @@ build_dynamic_cast_1 (tree type, tree expr, tsubst_flags_t complain)

  	  target_type = TYPE_MAIN_VARIANT (TREE_TYPE (type));
  	  static_type = TYPE_MAIN_VARIANT (TREE_TYPE (exprtype));
+
+	  if ((CLASSTYPE_FINAL (static_type)
+	       && !DERIVED_FROM_P (target_type, static_type))
+	      || (CLASSTYPE_FINAL (target_type)
+		  && !DERIVED_FROM_P (static_type, target_type)))
+	    {
+	      if (complain & tf_warning)
+		{
+		  if (VAR_P (old_expr))
+	            warning (0, "dynamic_cast of %q#D to %q#T can never succeed",
+				old_expr, type);
+		  else
+	            warning (0, "dynamic_cast of %q#E to %q#T can never succeed",
+				old_expr, type);
+		}
+
+	      if (tc == REFERENCE_TYPE)
+		{
+		  tree expr = throw_bad_cast ();
+		  TREE_TYPE (expr) = type;
+		  return expr;
+		}
+	      else
+		return build_zero_cst (type);
+	    }
+
  	  td2 = get_tinfo_decl (target_type);
  	  if (!mark_used (td2, complain) && !(complain & tf_error))
  	    return error_mark_node;
diff --git a/gcc/testsuite/g++.dg/rtti/dyncast8.C b/gcc/testsuite/g++.dg/rtti/dyncast8.C
new file mode 100644
index 0000000..f63ed5d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/rtti/dyncast8.C
@@ -0,0 +1,53 @@
+// PR c++/12277
+// { dg-do compile { target c++11 } }
+// { dg-additional-options "-fdump-tree-original" }
+
+struct A1 { virtual ~A1 () { } };
+struct A2 { virtual ~A2 () { } };
+
+struct B1 { virtual ~B1 () { } };
+struct B2 final : B1 { virtual ~B2 () { } };
+
+struct C1 { virtual ~C1 () { } };
+struct C2 final { virtual ~C2 () { } };
+
+A1 *a1;
+
+B1 *b1;
+B2 *b2;
+
+C1 *c1;
+C2 *c2;
+
+void
+foo (void)
+{
+  {
+    A2 *x = dynamic_cast<A2 *> (a1);
+  }
+
+  {
+    B2 *x = dynamic_cast<B2 *> (b1);
+    // The following cast may throw bad_cast.
+    B2 &y = dynamic_cast<B2 &> (*b1);
+  }
+
+  {
+    B1 *x = dynamic_cast<B1 *> (b2);
+    B1 &y = dynamic_cast<B1 &> (*b2);
+  }
+
+  {
+    C2 *x = dynamic_cast<C2 *> (c1); // { dg-warning "can never succeed" }
+    // The following cast may throw bad_cast.
+    C2 &y = dynamic_cast<C2 &> (*c1); // { dg-warning "can never succeed" }
+  }
+
+  {
+    C1 *x = dynamic_cast<C1 *> (c2); // { dg-warning "can never succeed" }
+    // The following cast may throw bad_cast.
+    C1 &y = dynamic_cast<C1 &> (*c2); // { dg-warning "can never succeed" }
+  }
+}
+
+// { dg-final { scan-tree-dump-times "bad_cast" 3 "original" } }
-- 
2.7.0.240.g19e8eb6