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Showing content from https://cplusplus.github.io/LWG/issue3300 below:

Non-array ssize overload is underconstrained

This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of C++20 status.

Section: 24.7 [iterator.range] Status: C++20 Submitter: Casey Carter Opened: 2019-09-27 Last modified: 2021-02-25

Priority: 3

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Discussion:

The overload of ssize specified in 24.7 [iterator.range]/18 has no constraints, yet it specializes make_signed_t which has a precondition that its type parameter is an integral type or enumeration but not bool (21.3.9.4 [meta.trans.sign]). This precondition needs to be propagated to ssize as "Mandates [or Constraints]: decltype(c.size()) [meets the requirements for the type argument to make_signed]". "Mandates" seems to be more in line with LWG guidance since there are no traits nor concepts that observe ssize.

[2019-11-16 Issue Prioritization]

Priority to 3 after reflector discussion.

Previous resolution [SUPERSEDED]:

This wording is relative to N4830.

1. Modify 24.7 [iterator.range] as indicated:

   template<class C> constexpr auto ssize(const C& c)
     -> common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>;
   

   -?- Mandates: decltype(c.size()) is a (possibly cv-qualified) integral or enumeration type but not a bool type.

   -18- Returns:

   static_cast<common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>>(c.size())
   

[2019-10-28; Tim provides improved wording]

Previous resolution [SUPERSEDED]:

This wording is relative to N4835.

1. Modify 24.7 [iterator.range] as indicated:

   template<class C> constexpr auto ssize(const C& c)
     -> common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>;
   

   -?- Mandates: decltype(c.size()) is an integral or enumeration type other than bool.

   -18- Returns:

   static_cast<common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>>(c.size())
   

[2019-11-18; Casey comments and improves wording]

It would be better to provided the Mandates: guarantee in [tab:meta.trans.sign] instead of one special place where the make_signed template is used. The wording below attempts to realize that.

[2019-11-23 Issue Prioritization]

Status to Tentatively Ready after five positive votes on the reflector.

Proposed resolution:

This wording is relative to N4835.

1. Change Table 52 — "Sign modifications" in [tab:meta.trans.sign] as indicated:

   Table 52 — Sign modifications [tab:meta.trans.sign] Template Comments template <class T>
struct make_signed; If T names a (possibly cv-qualified) signed integer type (6.9.2 [basic.fundamental]) then
   the member typedef type names the type T; otherwise, if T names a
   (possibly cv-qualified) unsigned integer type then type names the
   corresponding signed integer type, with the same cv-qualifiers as T;
   otherwise, type names the signed integer type with smallest
   rank (6.9.6 [conv.rank]) for which sizeof(T) == sizeof(type), with the same
   cv-qualifiers as T.
   Requires Mandates: T shall be is a n (possibly cv-qualified) integral type or enumeration type other than cv but not abool type. template <class T>
struct make_unsigned; If T names a (possibly cv-qualified) unsigned integer type (6.9.2 [basic.fundamental]) then
   the member typedef type names the type T; otherwise, if T names a
   (possibly cv-qualified) signed integer type then type names the
   corresponding unsigned integer type, with the same cv-qualifiers as T;
   otherwise, type names the unsigned integer type with smallest
   rank (6.9.6 [conv.rank]) for which sizeof(T) == sizeof(type), with the same
   cv-qualifiers as T.
   Requires Mandates: T shall be is a n (possibly cv-qualified) integral type or enumeration type other than cv but not abool type.

2. Change 24.7 [iterator.range] as indicated:

   template<class C> constexpr auto ssize(const C& c)
     -> common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>;
   

   -18- Returns Effects: Equivalent to:

   return static_cast<common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>>(c.size());
   

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