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Re: Array size validity and SFINAE

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Andrey Tarasevich <noone@noone.net> writes:

> On Mon 3/2/2026 12:38 AM, Tim Rentsch wrote:
>
>> First, the struct S is irrelevant;  it has no bearing on the
>> question or the observed behavior.
>
> You are right.  However, the `foo<int>` call in my code is a remnant of
> an experiment with non-existing nested name.  It also illustrates the
> problem.
>
> Yet my original intent was to use
>
>    foo<S>();
>
> call.
>
>> Third, if we add a statement
>>
>>      int fred = T::N;
>>
>> to the first template, gcc gives an error, whereas clang still
>> outputs '...'.
>
> Add where exactly?  Into the body of the function template?

Yes, just before the std::cout line.

> That would
> immediately take us out of SFINAE territory and make it a hard error
> in cases where `T::N` does not exist.

Oh.  I still think of that as SFINAE, since clang does the right
thing. 

> Not surprisingly, any compiler
> that chooses the first overload will subsequently issue an error.

Right.

>> 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 '...'.
>
> That is another facet of the problem I mentioned in my original
> question, since array declaration with negative array size is supposed
> to be invalid.
>
> Not sure what importance you assign to that "after adding..." part,
> since adding 'int fred = ...' to the first template is completely
> beside the point in this case.

It's a way of showing gcc is confused, since there is another
template that doesn't have an error, which clang correctly
recognizes.

>> 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'.
>
> This is expected, of course.  Again, adding 'int fred = ...' to the
> first template has no bearing on it.

Yes, expected.  The point of having 'int fred = ...' is to
emphasize the distinction between gcc and clang considering
the pair of invocations foo<S>(0) and foo<U>(0).

>> 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.
>
> Yes, in ordinary functions, as opposed to function templates, GCC does
> not ignore any issues with array sizes in parameter declarations,
> which is the reason I specifically mentioned that the issue is only
> visible in template context.

I didn't really think any of my reporting would surprise you.  I
included everything in the interest of completeness and to
illustrate my thought processes.

After seeing your reply I tried one more test, changing the first
template to

  template <typename T> void foo(int (*p)[T::N])
  {
    std::cout << "array of size " << sizeof *p << std::endl;
  }

in which case the invocation

    foo<S>(0);

produces '...' under both gcc and clang.

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Array size validity and SFINAE Andrey Tarasevich <noone@noone.net> - 2026-02-25 08:08 -0800
  Re: Array size validity and SFINAE Marcel Mueller <news.5.maazl@spamgourmet.org> - 2026-03-01 16:55 +0100
    Re: Array size validity and SFINAE Andrey Tarasevich <noone@noone.net> - 2026-03-01 08:23 -0800
      Re: Array size validity and SFINAE "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-03-01 16:36 -0800
        Re: Array size validity and SFINAE Andrey Tarasevich <noone@noone.net> - 2026-03-01 16:45 -0800
    Re: Array size validity and SFINAE Bonita Montero <Bonita.Montero@gmail.com> - 2026-03-02 13:21 +0100
  Re: Array size validity and SFINAE Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-03-02 00:38 -0800
    Re: Array size validity and SFINAE Andrey Tarasevich <noone@noone.net> - 2026-03-02 07:48 -0800
      Re: Array size validity and SFINAE Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-03-02 21:34 -0800

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