[Adopted as a DR as part of paper P0588R1 at the October, 2018 meeting.]

According to 9.3.4.7 [dcl.fct.default] paragraph 7,

A local variable shall not appear as a potentially-evaluated expression in a default argument.

This prohibits plausible uses of constexpr and static local variables. Presumably this rule should be similar to the one in 11.6 [class.local] paragraph 1, regarding local classes, which applies to odr-use, not potential evaluation, and to variables with automatic storage duration.