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### 902. Regular is the wrong concept to constrain numeric_limits

Section: 17.3.5 [numeric.limits] Status: NAD Concepts Submitter: Alisdair Meredith Opened: 2008-09-24 Last modified: 2017-06-15

Priority: Not Prioritized

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Discussion:

Addresses FR 32 and DE 16

`numeric_limits` has functions specifically designed to return NaNs, which break the model of `Regular` (via its axioms.) While floating point types will be acceptible in many algorithms taking `Regular` values, it is not appopriate for this specific API and we need a less refined constraint.

FR 32:

> The definition of `numeric_limits<>` as requiring a regular type is both conceptually wrong and operationally illogical. As we pointed before, this mistake needs to be corrected. For example, the template can be left unconstrained. In fact this reflects a much more general problem with concept_maps/axioms and their interpretations. It appears that the current text heavily leans toward experimental academic type theory.

DE 16:

> The class template `numeric_limits` should not specify the Regular concept requirement for its template parameter, because it contains functions returning NaN values for floating-point types; these values violate the semantics of EqualityComparable.

[ Summit: ]

> Move to Open. Alisdair and Gaby will work on a solution, along with the new treatment of axioms in clause 14.

Proposed resolution: