This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115d. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2024-10-26
[Accepted as a DR at the November, 2023 meeting.]
Presumably something like the following should be well-formed, where a deduction failure in a partial specialization is handled as a SFINAE case as it is with function templates and not a hard error:
template <class T, class U> struct X { typedef char member; }; template<class T> struct X<T, typename enable_if<(sizeof(T)>sizeof( float)), float>::type> { typedef long long member; }; int main() { cout << sizeof(X<double, float>::member); }
However, this does not appear to be described anywhere in the Standard.
Additional notes (January, 2023)
The section on SFINAE (13.10.3.1 [temp.deduct.general] paragraph 8) is not specific to function templates, and 13.7.6.2 [temp.spec.partial.match] paragraph 2 hands off the "matching" determination for partial specializations to 13.10.3 [temp.deduct] in general. However, the definition of deduction substitution loci in 13.10.3.1 [temp.deduct.general] paragraph 7 does not account for the template argument list of a partial specialization.
Proposed resolution (approved by CWG 2023-11-08):
Change in 13.10.3.1 [temp.deduct.general] paragraph 7 as follows:
The deduction substitution loci areThe substitution occurs in all types and expressions that are used in the deduction substitution loci. ...
- the function type outside of the noexcept-specifier,
- the explicit-specifier,
and- the template parameter declarations
., and- the template argument list of a partial specialization (13.7.6.1 [temp.spec.partial.general]).