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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Previous resolution [SUPERSEDED]: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.
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Previous resolution [SUPERSEDED]: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.
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.
Change Table 52 — "Sign modifications" in [tab:meta.trans.sign] as indicated:
| 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. |
|
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. |
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());