Rationale (September, 2013):

Arrays of runtime bound were moved from the normative specification to a proposed Technical Specification.

The expansion of a range-based for in 8.6.5 [stmt.ranged] paragraph 1 involves a declaration of the form

  auto && __range = range-init;

However, it is not permitted to bind a reference to an array of runtime bound (9.3.4.3 [dcl.ref] paragraph 5), even though it is intended that such arrays can be used in a range-based for.