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Showing content from https://cplusplus.github.io/LWG/issue2197 below:

Specification of is_[un]signed unclear for non-arithmetic types

This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of C++14 status.

Section: 21.3.6.4 [meta.unary.prop] Status: C++14 Submitter: Daniel Krügler Opened: 2012-10-07 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

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

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.

[2013-03-15 Issues Teleconference]

Moved to Tentatively Ready.

[2013-04-20 Bristol]

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; Ifis_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; Ifis_arithmetic<T>::value && is true, the same result as
   integral_constant<bool, T(0) < T(-1) >::value ;
   otherwise, false.  

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