This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 108h. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
[Adopted at the February, 2016 meeting.]
According to 9.2.6 [dcl.constexpr] paragraph 6,
If the instantiated template specialization of a constexpr function template or member function of a class template would fail to satisfy the requirements for a constexpr function or constexpr constructor, that specialization is still a constexpr function or constexpr constructor, even though a call to such a function cannot appear in a constant expression.
The restriction on appearing in a constant expression assumes the previous wording that made such a specialization non-constexpr, and a call to a non-constexpr function cannot appear in a constant expression. With the current wording, however, there is no normative restriction against calls to such specializations. 7.7 [expr.const] should be updated to include such a prohibition.
Proposed resolution (January, 2016):
Add the following bullet following 7.7 [expr.const] bulllet 2.3:
A conditional-expression e is a core constant expression unless the evaluation of e, following the rules of the abstract machine (6.9.1 [intro.execution]), would evaluate one of the following expressions:
an invocation of an undefined constexpr function or an undefined constexpr constructor;