Proposed resolution (August, 2017):

Change 7.6.1.9 [expr.static.cast] paragraph 10 as follows:

A value of integral or enumeration type can be explicitly converted to a complete enumeration type. The If the enumeration type has a fixed underlying type, the value is first converted to that type by integral conversion, if necessary, and then to the enumeration type. If the enumeration type does not have a fixed underlying type, the value is unchanged if the original value is within the range of the enumeration values (9.8.1 [dcl.enum]). Otherwise, and otherwise, the behavior is undefined.