This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 112c. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
[Accepted as a DR at the March, 2018 (Jacksonville) meeting.]
It is not clear whether a constexpr function can be a vararg function or not. In particular, it is unclear if va_list is a literal type and whether va_start, va_arg, and va_end produce constant expressions.
Proposed resolution (November, 2017)
Add a new bullet to the list in 7.7 [expr.const] paragraph 2, and update the text as follows:
An 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:
a throw-expression (7.6.18 [expr.throw])
If e satisfies the constraints of a core constant expression, but evaluation of e would evaluate an operation that has undefined behavior as specified in Clause 16 [library] through Clause 33 [thread] of this document, it is unspecified whether e is a core constant expression.