[c++1z] N4295: fold-expressions.

This is a new form of expression of the form:

  (expr op ... op expr)

where one of the exprs is a parameter pack. It expands into

  (expr1 op (expr2onwards op ... op expr))

(and likewise if the pack is on the right). The non-pack operand can be
omitted; in that case, an empty pack gives a fallback value or an error,
depending on the operator.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221573 91177308-0d34-0410-b5e6-96231b3b80d8