On 10/11/19 6:31 PM, Jason Merrill wrote:
> On 10/10/19 2:06 PM, Bernd Edlinger wrote:
>> On 10/10/19 7:49 PM, Jason Merrill wrote:
>>> On 10/10/19 10:42 AM, Bernd Edlinger wrote:
>>>> Hi,
>>>>
>>>> this fixes a crash when -Wshadow=compatible-local is
>>>> enabled in the testcase g++.dg/parse/crash68.C
>>>
>>> Why does that flag cause this crash?
>>>
>>
>> gcc/cp/name-lookup.c:
>>
>>        if (warn_shadow)
>>          warning_code = OPT_Wshadow;
>>        else if (warn_shadow_local)
>>          warning_code = OPT_Wshadow_local;
>>        else if (warn_shadow_compatible_local
>>                 && (same_type_p (TREE_TYPE (old), TREE_TYPE (decl))
>>                     || (!dependent_type_p (TREE_TYPE (decl))
>>                         && !dependent_type_p (TREE_TYPE (old))
>>                         /* If the new decl uses auto, we don't yet know
>>                            its type (the old type cannot be using auto
>>                            at this point, without also being
>>                            dependent).  This is an indication we're
>>                            (now) doing the shadow checking too
>>                            early.  */
>>                         && !type_uses_auto (TREE_TYPE (decl))
>>                         && can_convert (TREE_TYPE (old), TREE_TYPE (decl),
>>                                         tf_none))))
>>          warning_code = OPT_Wshadow_compatible_local;
>>
>> if -Wshadow=compatible-local is used, the can_convert function crashes
>> in instantiate_class_template_1.
> 
> Right, checking can_convert is problematic here, as it can cause template instantiations that change the semantics of the program.  Or, in this case, crash.
> 

So I try to make C++ behave more consistently with the code in c-decl.c,
thus dependent on warn_shadow but not on warn_shadow_local and/or
warn_shadow_compatible_local:

           if (warn_shadow)
              warning_code = OPT_Wshadow;
            else if (comptypes (TREE_TYPE (old_decl), TREE_TYPE (new_decl)))
              warning_code = OPT_Wshadow_compatible_local;
            else
              warning_code = OPT_Wshadow_local;
            warned = warning_at (DECL_SOURCE_LOCATION (new_decl), warning_code,
                                 "declaration of %qD shadows a parameter",
                                 new_decl);

I cannot remove the if (warn_shadow) since this breaks gcc.dg/pr48062.c
which uses:

#pragma GCC diagnostic ignored "-Wshadow"

to disable a -Wshadow=compatible-local warning, but while -Wno-shadow on the
command line disables also dependent warnings the pragma does not (always) do that.

So instead I'd like to adjust the doc of -Wshadow to reflect the implementation
and remove the if(warn_shadow_local) to have C and C++ behave identical and
hopefully now in sync with the doc.


Bootstrapped and reg-tested on x86_64-pc-linux-gnu.
Is it OK for trunk?


Thanks
Bernd.