N4296 5.3.3 [expr.sizeof] says

The sizeof operator shall not be applied to an expression that has function or incomplete type, to an enumeration type whose underlying type is not fixed before all its enumerators have been declared, to the parenthesized name of such types, or to a glvalue that designates a bit-field.

Now that "an enumeration type whose underlying type is not fixed before all its enumerators have been declared" is considered as an incomplete type (7.2[dcl.enum]p6), to an enumeration type whose underlying type is not fixed before all its enumerators have been declared become redundant.