This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 112e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
[Accepted as a DR at the June, 2023 meeting.]
Subclause 13.8.1 [temp.res.general] paragraph 6 specifies rules to determine when a template is valid, but the specification is in terms of "instantiation". However, some kinds of templates (namely alias templates) are not instantiated.
Further, the rule discusses only templates, but should apply to any templated entity.
Proposed resolution (approved by CWG 2023-05-12):
Change in 13.8.1 [temp.res.general] paragraph 6 as follows:
The validity of a
templatemay be checked prior to any instantiation. [Note 3: Knowing which names are type names allows the syntax of every template to be checked in this way. —end note]
The program is ill-formed, no diagnostic required, if:
- no valid specialization, ignoring static_assert-declarations that fail , can be generated for a
templateor a substatement of a constexpr if statement (8.5.2 [stmt.if]) within a templateand the template is not instantiated, or
- any constraint-expression in the program, introduced or otherwise, has (in its normal form) an atomic constraint A where no satisfaction check of A could be well-formed and no satisfaction check of A is performed, or
- every valid specialization of a variadic template requires an empty template parameter pack, or
- a hypothetical instantiation of a
templateimmediately following its definition would be ill-formed due to a construct that does not depend on a template parameter, or
- the interpretation of such a construct in the hypothetical instantiation is different from the interpretation of the corresponding construct in any actual instantiation of the