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

[Patrick Palka <patrick at parcs dot ath dot cx>]

On Wed, Feb 3, 2016 at 1:40 PM, Patrick Palka <patrick@parcs.ath.cx> wrote:
> 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);
> +               }

It just occurred to me that issuing this warning during template
instantiation may be undesirable if the dynamic_cast being built was
originally dependent (for the same reasons it is undesirable to issue
-Wuseless-cast warnings during instantiation, which we already avoid
doing).  Especially so since this particular warning is not associated
with any warning flag.  I'll try to come up with a more careful patch
for stage 1.