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2422.std::numeric_limits<T>::is_modulo description: "most machines" errata

Section: 17.3.5.2[numeric.limits.members]Status:C++17Submitter: Melissa MearsOpened: 2014-08-06Last modified: 2017-07-30

Priority:2

View all otherissues in [numeric.limits.members].

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Discussion:

The seemingly non-normative (?) paragraph 61 (referring to N3936) describing how "most machines" definestd::numeric_limits<T>::is_modulo in [numeric.limits.members] appears to have some issues, in my opinion.

-61- On most machines, this isfalse for floating types,true for unsigned integers, andtrue for signed integers.

Issues I see:

1. Very minor: change clause 2 to "this isfalse for floatingpoint types". Other uses of the term say "floating point types" rather than just "floating types" — see nearbyis_iec559,tinyness_before, etc.

2. is_modulomust betrue for unsigned integers in order to be compliant with the Standard; this is not just for "most machines". For reference, this requirement is from [basic.fundamental] paragraph 4 with its footnote 48.

3. Depending on the definition of "most machines",is_modulo could befalse for most machines' signed integer types. GCC, Clang and Visual Studio, the 3 most popular C++ compilers by far, by default treat signed integer overflow as undefined.

As an additional note regarding the definition ofis_modulo, it seems like it should be explicitly mentioned that on an implementation for which signed integer overflow is undefined,is_modulo shall befalse for signed integer types. It took bugs filed for all three of these compilers before they finally changed (or planned to change)is_modulo tofalse for signed types.

[2014-12 telecon]

HH: agree with the proposal, don't like the phrasing
AM: second note feels a bit wooly
WB: not even happy with the first note, notes shouldn't say "shall"
JW: the original isn't very prescriptive because "on most machines" is not something the standard controls.
AM: "On most machines" should become a note too?
AM: first note is repeating something defined in core, shouldn't say it normatively here. Change "shall" to "is"?
MC: don't like "signed integer overflow is left undefined" ... it's just plain undefined.
AM: implementations can define what they do in that case and provide guarantees.
WB: in paragraph 61, would like to see "this" replaced by "is_modulo"
AM: Move to Open

[2015-05-05 Lenexa]

Marshall: I will contact the submitter to see if she can re-draft the Proposed Resolution

Previous resolution [SUPERSEDED]:

1. Edit 17.3.5.2[numeric.limits.members] around p60 as indicated:

   static constexpr bool is_modulo;

   -60- True if the type is modulo.(footnote) A type is modulo if, for any operation involving+,-, or* on values of that type whose result would fall outside the range[min(), max()], the value returned differs fromthe true value by an integer multiple ofmax() - min() + 1.

   -??- [Note:is_modulo shall betrue for unsigned integer types (6.9.2[basic.fundamental]). —end note]

   -??- [Note:is_modulo shall befalse for types for which overflow is undefined on the implementation, because such types cannot meet the modulo requirement. Often, signed integer overflow is left undefined on implementations. —end note]

   -61- On most machines, this isfalse for floatingpoint types,true for unsigned integers, andtrue for signed integers.

   -62- Meaningful for all specializations.

[2016-05-21 Melissa Mears comments and provides improved wording]

GCC and Clang have-fwrapv and MSVC (as of a May 2016 preview build)has/d2UndefIntOverflow- as compiler command line flags to definesigned integer overflow as wrapping. Such implementations can't setis_modulo totrue for signed integer types when these options areenabled, because if translation units using opposite settings werelinked together, the One Definition Rule would be violated.

Previous resolution [SUPERSEDED]:

1. Edit 17.3.5.2[numeric.limits.members] around p60 as indicated:

   static constexpr bool is_modulo;

   -60- True if the type is modulo.(footnote) A type is modulo if, for any operation involving+,-, or* on values of that type whose result would fall outside the range[min(), max()], the value returned differs from the true value by an integer multiple ofmax() - min() + 1.

   -??- [Note:is_modulo istrue for unsigned integer types (6.9.2[basic.fundamental]). —end note]

   -??- [Note:is_modulo isfalse for signed integer types (6.9.2[basic.fundamental]) unless an implementation, as an extension to this International Standard, defines signed integer overflow to wrap asspecified by the reference mentioned in footnote 217. —end note]

   -??- [Note:is_modulo isfalse for floating point types on most machines. —end note]

   -61- On most machines, this isfalse for floating types,true for unsigned integers, andtrue for signed integers.

   -62- Meaningful for all specializations.

[2016-06, Oulu]

We believe that the notes about unsigned integer types and floating point types are already evident from whatthe standard describes and should be removed. Furthermore the suggested note for signed integer types really shouldn't refer to afootnote in the own document, because footnotes have no stable identifiers. The below given revised resolutionhas been changed accordingly.

[2016-06 Oulu]

Added rationalization for changes. Moved to Ready.

Friday: status to Immediate

Proposed resolution:

1. Edit 17.3.5.2[numeric.limits.members] around p60 as indicated:

   static constexpr bool is_modulo;

   -60- True if the type is modulo.(footnote) A type is modulo if, for any operation involving+,-, or* on values of that type whose result would fall outside the range[min(), max()], the value returned differs from the true value by an integer multiple ofmax() - min() + 1.

   -??- [Example:is_modulo isfalse for signed integer types (6.9.2[basic.fundamental]) unless an implementation, as an extension to this International Standard, defines signed integer overflow to wrap. —end example]

   -61- On most machines, this isfalse for floating types,true for unsigned integers, andtrue for signed integers.

   -62- Meaningful for all specializations.