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;

• an invocation of an instantiated constexpr function or constexpr constructor that fails to satisfy the requirements for a constexpr function or constexpr constructor (9.2.6 [dcl.constexpr]);

• ...

[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.