Proposed resolution (November, 2017)

1. 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 relational (7.6.9 [expr.rel]) or equality (7.6.10 [expr.eq]) operator where the result is unspecified; or

• a throw-expression (7.6.18 [expr.throw]). ; or

• an invocation of the va_arg macro (17.13.2 [cstdarg.syn]).

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 32 [thread] of this document, or an invocation of the va_start macro (17.13.2 [cstdarg.syn]), it is unspecified whether e is a core constant expression.

[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.