Proposed resolution:

This wording is relative to N3376.

1. Change Table 49 as indicated:

   Table 49 — Type property predicates

   Template	Condition	Preconditions
   template <class T> struct is_signed;	If is_arithmetic<T>::value && is true, the same result as integral_constant<bool, T(-1) < T(0)>::value; otherwise, false.
   template <class T> struct is_unsigned;	If is_arithmetic<T>::value && is true, the same result as integral_constant<bool, T(0) < T(-1)>::value; otherwise, false.

[ 2013-04-20 Bristol ]

[ 2013-03-15 Issues Teleconference ]

Moved to Tentatively Ready.

The pre-conditions for the trait is_signed allow for any types as template arguments, including non-arithmetic ones.

But the current wording in Table 49 defining the predicate condition,

is_arithmetic<T>::value && T(-1) < T(0)

looks like real code and so leaves it open whether such argument types would create a well-formed instantiation of the trait template or not. As written this definition would lead to a hard instantiation error for a non-arithmetic type like e.g.

struct S {};

I would suggest that the wording clarifies that the instantiation would be valid for such types as well, by means of a specification that is not an exact code pattern. This also reflects how existing implementations behave.