Re: [PATCH] accept all C integer types in function parameters referenced by alloc_align (PR 88363)

[Martin Sebor <msebor@gmail.com>]

On 12/11/18 3:52 PM, Marek Polacek wrote:
> On Tue, Dec 11, 2018 at 03:46:37PM -0700, Martin Sebor wrote:
>> On 12/11/18 1:47 PM, Jakub Jelinek wrote:
>>> On Tue, Dec 11, 2018 at 01:36:58PM -0700, Martin Sebor wrote:
>>>> Attached is an updated version of the patch that restores
>>>> the original behavior for the positional argument validation
>>>> (i.e., prior to r266195) for integral types except bool as
>>>> discussed.
>>>
>>> I thought Jason wanted to also warn for scoped enums in C++.
>>
>> I missed that.  It seems needlessly restrictive to me to reject
>> the preferred kind of an enum when ordinary enums are accepted.
>> Jason, can you confirm that you really want a warning for B
>> below when there is none for A (GCC 8 doesn't complain about
>> either, Clang complains about both, ICC about neither when
>> using alloc_size -- it doesn't understand alloc_align):
>>
>>    enum A { /* ... */ };
>>    __attribute__ ((alloc_align (1))) void* f (A);
>>
>>    enum class B { /* ... */ };
>>    __attribute__ ((alloc_align (1))) void* g (B);
>>
>> The only use case I can think of for enums is in APIs that try
>> to restrict the available choices of alignment to those of
>> the enumerators.  In that use case, I would expect it to make
>> no difference whether the enum is ordinary or the scoped kind.
> 
> The reason was that C++ scoped enumerations don't implicitly convert to
> integral types.

I'm not sure we're talking about the same thing.  There is no
conversion in the use case I described, the attribute argument
just refers to the function parameter, and the function is called
with an argument of the enumerated type of the parameter.  Like
this:

   enum class Alignment { a4 = 4, a8 = 8 };

   __attribute__ ((alloc_align (1))) void*
   aligned_alloc (Alignment, size_t);

   void *p = aligned_alloc (Alignment::a8, 32);

My question is: if we think it makes sense to accept this use
case with ordinary enums why would we not want to make it possible
with scoped enums?  People tend to think of the latter as preferable
over the former.

Martin