CWG3003 Naming a deducible template for class template argument deduction

[jensmaurer]

The core language specification is clear that dependent names cannot designate templates subject to class template argument deduction, thus excluding the names of template template parameters.

However, https://wg21.link/range.utility.conv.to#4 relies on such a non-existing facility.

CWG3003 seeks to add a note to highlight the status quo of the core language specification.

LWG4381 asks for removal of that unimplementable feature.

If (L)EWG wishes to add a feature to the core language that makes https://wg21.link/range.utility.conv.to#4 valid (possibly limited to situations similar to the first example in CWG3003, excluding the second example), a paper would be needed.

[jfbastien]

Discussed with chairs, LEWG will see first if they need it, and ask EWG for it if so.

[je4d]

EWG discussed this in Kona on Friday afternoon along with the proposed fixes in #2467 and #2469, and took the following poll:

CWG3003: EWG wants to keep ranges::to as currently specified, with the understanding that specification is incomplete for CTAD in template parameters, to be fixed later

SF	F	N	A	SA
20	11	1	0	0

consensus.