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

When either the static type or the target type of a dynamic cast is
marked 'final', and the type marked final is not derived from the other,
then there is no way to declare a class that is derived from both types
(since one of the types is marked final) so there is no way for such a
dynamic cast to possibly succeed.

This patch detects this case and emits a warning accordingly.

Bootstrap + regtest in progress. Is this OK to commit if testing
succeeds?

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                        | 18 ++++++++++++++
 gcc/testsuite/g++.dg/rtti/dyncast8.C | 47 ++++++++++++++++++++++++++++++++++++
 2 files changed, 65 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/rtti/dyncast8.C

On Sun, Nov 8, 2015 at 10:30 PM, Patrick Palka <patrick@parcs.ath.cx> wrote:
> When either the static type or the target type of a dynamic cast is
> marked 'final', and the type marked final is not derived from the other,
> then there is no way to declare a class that is derived from both types
> (since one of the types is marked final) so there is no way for such a
> dynamic cast to possibly succeed.
>
> This patch detects this case and emits a warning accordingly.
>
> Bootstrap + regtest in progress. Is this OK to commit if testing
> succeeds?
>
> 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                        | 18 ++++++++++++++
>  gcc/testsuite/g++.dg/rtti/dyncast8.C | 47 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 65 insertions(+)
>  create mode 100644 gcc/testsuite/g++.dg/rtti/dyncast8.C
>
> diff --git a/gcc/cp/rtti.c b/gcc/cp/rtti.c
> index be25be4..b823f8b 100644
> --- a/gcc/cp/rtti.c
> +++ b/gcc/cp/rtti.c
> @@ -698,6 +698,24 @@ 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);
> +               }
> +             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..d98878c
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/rtti/dyncast8.C
> @@ -0,0 +1,47 @@
> +// PR c++/12277
> +// { dg-do require { target c++11 } }
> +
> +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); // { dg-bogus "can never succeed" }
> +  }
> +
> +  {
> +    B2 *x = dynamic_cast<B2 *> (b1); // { dg-bogus "can never suceed" }
> +    B2 &y = dynamic_cast<B2 &> (*b1); // { dg-bogus "can never suceed" }
> +  }
> +
> +  {
> +    B1 *x = dynamic_cast<B1 *> (b2); // { dg-bogus "can never suceed" }
> +    B1 &y = dynamic_cast<B1 &> (*b2); // { dg-bogus "can never suceed" }
> +  }
> +
> +  {
> +    C2 *x = dynamic_cast<C2 *> (c1); // { dg-warning "can never succeed" }
> +    C2 &y = dynamic_cast<C2 &> (*c1); // { dg-warning "can never suceed" }
> +  }
> +
> +  {
> +    C1 *x = dynamic_cast<C1 *> (c2); // { dg-warning "can never succeed" }
> +    C1 &y = dynamic_cast<C1 &> (*c2); // { dg-warning "can never suceed" }
> +  }
> +}
> --
> 2.6.3.412.gac8e876.dirty
>

Oops, this is the wrong test case (forgot to amend the patch before
sending).  The correct test case has s/require/compile/ and
s/suceed/succeed/g done on it.

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.

Jason