This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 117b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2025-08-11
1495. Partial specialization of variadic class templateSection: 13.7.6 [temp.spec.partial] Status: CD3 Submitter: Jason Merrill Date: 2012-04-16[Moved to DR at the April, 2013 meeting.]
Consider an example like
template <int B, typename Type1, typename... Types>
struct A;
template<typename... Types>
struct A<0, Types...> { };
A<0,int,int> t;
In this case, the partial specialization seems well-formed by the rules in 13.7.6 [temp.spec.partial], but it is not more specialized than the primary template. However, 13.7.6.2 [temp.spec.partial.match] says that if exactly one matching specialization is found, it is used, which suggests that the testcase is well-formed. That seems undesirable; I think a partial specialization that is not more specialized than the primary template should be ill-formed.
If the example is rewritten so that both versions are partial specializations, i.e.,
template <int B, typename... Types>
struct A;
template <int B, typename Type1, typename... Types>
struct A<B, Type1, Types...> { }
template<typename... Types>
struct A<0, Types...> { };
A<0,int,int> t;
There is implementation variance, with gcc and clang reporting an ambiguity and EDG choosing the second specialization.
Proposed resolution (October, 2012):
Add the following as a new bullet in 13.7.6.1 [temp.spec.partial.general] paragraph 9:
...
The argument list of the specialization shall not be identical to the implicit argument list of the primary template.
The specialization shall be more specialized than the primary template (13.7.6.3 [temp.spec.partial.order]).
...
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