Path: csiph.com!eternal-september.org!feeder.eternal-september.org!nntp.eternal-september.org!.POSTED!not-for-mail
From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c++
Subject: Re: Array size validity and SFINAE
Date: Mon, 02 Mar 2026 00:38:52 -0800
Organization: A noiseless patient Spider
Lines: 80
Message-ID: <86zf4qeicj.fsf@linuxsc.com>
References: <10nn6pp$su58$1@dont-email.me>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Injection-Date: Mon, 02 Mar 2026 08:39:08 +0000 (UTC)
Injection-Info: dont-email.me; posting-host="879346d16fc7c1cd3f18baf5c462c148"; logging-data="1046631"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1+58bB17pr4tof5Rrv/TuyIX642TYESOeY="
User-Agent: Gnus/5.11 (Gnus v5.11) Emacs/22.4 (gnu/linux)
Cancel-Lock: sha1:XPyh4oUELGLXDWuE93OfXDgzdDM= sha1:4QnGrRIf4np+xBVZ2rIFqK8ivRY=
Xref: csiph.com comp.lang.c++:123505

Andrey Tarasevich <noone@noone.net> writes:

>   #include <iostream>
>
>   struct S { static constexpr int N = -1; };
>
>   template <typename T> void foo(int [T::N])
>   {
>     std::cout << "array" << std::endl;
>   }
>
>   template <typename T> void foo(...)
>   {
>     std::cout << "..." << std::endl;
>   }
>
>   int main()
>   {
>     foo<int>(0);
>   }
>
> GCC outputs "array", Clang and MSVC output "..."
>
> The struct definition can be changed to `struct S {};` with the same
> behavior from GCC.  I.e. in template contexts GCC does not check the
> validity of the size specified in array parameter declaration:  it
> doesn't care whether it is positive, it doesn't even care whether the
> nested name exists.  One can guess that this is most likely a
> consequence of the array parameter type adjustment, which makes the
> size essentially "disappear".  However, it still does not necessarily
> mean that such array parameter declaration should be accepted as
> valid.
>
> So, who's right here standard-wise?

I certainly don't claim to be a C++ language lawyer;  at best I
might agree to be termed a language paralegal.  That said, here are
my thoughts.

First, the struct S is irrelevant;  it has no bearing on the
question or the observed behavior.

Second, clang is right, and gcc is wrong.  The output under gcc of
"array" would seem to imply that int::N exists, but it doesn't.

Third, if we add a statement

    int fred = T::N;

to the first template, gcc gives an error, whereas clang still
outputs '...'.  My understanding of SFINAE matches what clang
does, and does not match what gcc does.

Fourth, if after adding 'int fred = ...' to the first template, we
change the call in main() to

    foo<S>(0)

then gcc again outputs 'array', whereas clang outputs '...'.

Fifth, if after adding 'int fred = ...' to the first template, we
define a new struct

    struct U { static constexpr unsigned int N = -1; };

and change the invocation in main() to

    foo<U>(0)

then both gcc and clang both output 'array'.

Incidentally, trying to define an analogous pair of overloaded
functions simply rejects the definition with a negative value for
the extent of the array parameter.  That holds for both gcc and
clang.

In every case clang does what I expect, and is consistent with what
I understand about SFINAE.  I can't say the same for gcc.  So,
either my understanding of SFINAE is messed up, or gcc is confused
about something.