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3300. Non-array ssize overload is underconstrained

Section: 25.7 [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 25.7 [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 (21.3.8.4 [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Previous resolution [SUPERSEDED]: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 25.7 [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Previous resolution [SUPERSEDED]: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 25.7 [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 (6.8.2 [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 (6.8.6 [conv.rank]) for which sizeof(T) == sizeof(type), with the same
    cv-qualifiers as T.
    RequiresMandates: T shall beis an (possibly cv-qualified) integral type or enumeration type other than cv but not a bool type.
    template <class T>
    struct make_unsigned;         		If T names a (possibly cv-qualified) unsigned integer type (6.8.2 [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 (6.8.6 [conv.rank]) for which sizeof(T) == sizeof(type), with the same
    cv-qualifiers as T.
    RequiresMandates: T shall beis an (possibly cv-qualified) integral type or enumeration type other than cv but not a bool type.

  2. Change 25.7 [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- ReturnsEffects: Equivalent to: 

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