Proposed resolution:

This wording is relative to N5054.

1. Modify [meta.trans.sign] as indicated:

   Table 59 — Sign modifications [tab:meta.trans.sign]

   Template	Comments
   template <class T> struct make_signed;	If `T` is a (possibly cv-qualified) signed integer type ([basic.fundamental]) then the member typedef type denotes `T`; otherwise, if `T` is a (possibly cv-qualified) unsigned integer type then type denotes the corresponding signed integer type, with the same cv-qualifiers as `T`; otherwise, type denotes the signed integer type with smallest rank ([conv.rank]) for which `sizeof(T) == sizeof(type)`, with the same cv-qualifiers as `T`. Mandates: `T` is either an integral or enumeration type other than cv `bool` or an enumeration type whose underlying type is not `bool`.
   template <class T> struct make_unsigned;	If `T` is a (possibly cv-qualified) unsigned integer type ([basic.fundamental]) then the member typedef type denotes `T`; otherwise, if `T` is a (possibly cv-qualified) signed integer type then type denotes the corresponding unsigned integer type, with the same cv-qualifiers as `T`; otherwise, type denotes the unsigned integer type with smallest rank ([conv.rank]) for which `sizeof(T) == sizeof(type)`, with the same cv-qualifiers as `T`. Mandates: `T` is either an integral or enumeration type other than cv `bool` or an enumeration type whose underlying type is not `bool`.

The intent is for `make_signed` and `make_unsigned` to reject `bool`, which is enforced by their Mandates element. However, enumerations whose underlying type is `bool` are accepted (presumably by accident), with implementation divergence:

using SB = std::make_signed_t<bool>; // all reject
enum E : bool {};
using SE = std::make_signed_t<E>;    // libc++ rejects, others yield signed char