> I guess this makes (0) a null pointer constant. I think [conv.ptr] can be made clearer by adding "(possibly parenthesized)" to the wording.

I think the intent would be the opposite, and that compilers are actually too permissive with respect to [conv.ptr]. Looking at the last sentence in [expr.prim.parent] (https://eel.is/c++draft/expr.prim.paren):

"...can be used in exactly the same contexts as those where E can be used, and with the same meaning, except as otherwise indicated."

I think the intent of "except as otherwise indicated" encompasses the case of null pointer conversion. There are a few context in which a parenthesized expression has not the same meaning as the expression. On top of my head, I can think of 4 places in the standard where a parenthesized expression "(expr)" has not the same meaning as the operand expression "expr":

- in "decltype(expr)": if expr is an id expression,
   then it changes the type result type. (This
   case is unfortunate IMO, we really should
   have a distinct unevaluated operator that
   would make this distinction, but that would
   deserve a proposal on its own).

- in null pointer conversion [conv.ptr]: a
   zero valued literal is required.

- in null member pointer conversion
   [conv.mem]: a zero valued literal is required.

- when an object of comparison category type
   is an operand of equality, relational or
   spaceship operator [cmp.categories.pre]:
   the other operand is required to be a zero
   valued literal. See
https://eel.is/c++draft/cmp.categories#pre-3

But I agree there should be an editorial fix. An informal footnote could be added, like

"...for instance, such context include the null pointer conversion (including member pointer), decltype operand, and comparison with a comparison categories."

and may even specify explicitly that the expression "(0)" is ill formed when used as pointer or when comparing with the result of a spaceship operator.
 

Julien V.
 

On July 21, 2023 6:09:59 a.m. EDT, "Lénárd Szolnoki via Std-Discussion" <std-discussion_at_[hidden]> wrote:
>On Fri, 2023-07-21 at 14:48 +0500, language.lawyer--- via Std-
>Discussion wrote:
>> > Is a parenthesised integer literal with value zero a null pointer
>> > constant?
>> >
>> > The current wording doesn't seem to allow that. [conv.ptr] says
>> > that "a
>> > null pointer constant is an integer literal ([lex.icon]) with value
>> > zero ...", which should exclude (0).
>> >
>> > All of gcc, clang and MSVC accept `void * ptr = (0);`.
>>
>> https://timsong-cpp.github.io/cppwp/n4868/expr.prim.paren#1.sentence-2
>> : The parenthesized expression can be used in exactly the same
>> contexts as those where E can be used, and with the same meaning,
>> except as otherwise indicated.
>
>Thank you, I guess this makes (0) a null pointer constant. I think
>[conv.ptr] can be made clearer by adding "(possibly parenthesized)" to
>the wording. But this is purely editorial.
>
>Cheers,
>Lénárd
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