CWG Issue 70 (original) (raw)

This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 117a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-04-13

70. Is an array bound a nondeduced context?

Section: 13.10.3.6 [temp.deduct.type]Status: CD1Submitter: Jack RouseDate: 29 Sep 1998

[Moved to DR at 4/01 meeting.]

Paragraph 4 lists contexts in which template formals are not deduced. Were template formals in an expression in the array bound of an array type specification intentionally left out of this list? Or was the intent that such formals always be explicitly specified? Otherwise I believe the following should be valid:

template <int I> class IntArr {};

template <int I, int J>
void concat( int (&d)[I+J], const IntArr<I>& a, const IntArr<J>& b ) {}

int testing()
{
    IntArr<2> a;
    IntArr<3> b;
    int d[5];

    concat( d, a, b );
}

Can anybody shed some light on this?

From John Spicer:

Expressions involving nontype template parameters are nondeduced contexts, even though they are omitted from the list in 13.10.3.6 [temp.deduct.type] paragraph 4. See 13.10.3.6 [temp.deduct.type] paragraphs 12-14:

  1. A template type argument cannot be deduced from the type of a non-type_template-argument_.
    ...
  2. If, in the declaration of a function template with a non-type template-parameter, the non-type template-parameter_is used in an expression in the function parameter-list, the corresponding_template-argument must always be explicitly specified or deduced elsewhere because type deduction would otherwise always fail for such a template-argument.

Proposed resolution (04/01): In 13.10.3.6 [temp.deduct.type] paragraph 4, add a third bullet: