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3614. iota_view::size and the most negative signed integer values

Section: 25.6.4.2 [range.iota.view] Status: New Submitter: Jiang An Opened: 2021-10-01 Last modified: 2021-10-14

Priority: 3

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Discussion:

According to 25.6.4.2 [range.iota.view]/15, when both W and Bound are integer-like, the expression in the return statement uses -value_ and -bound_. These operations result in undefined behavior when - is applied to the most negative integer value of a promoted type.

I believe that we can simply convert value_ and bound_ to the return type (make-unsigned-like-t<common_type_t<W, Bound>>) and then perform the subtraction. Such method should give the same results with UB eliminated.

Additionally, if we decide that iota_view<uint8_t, uint8_t>(uint8_t(1)).size() is well-defined (LWG 3597⁽ⁱ⁾), it should give the correct result. We can truncate the result to fit the type W.

[2021-10-14; Reflector poll]

Set priority to 3 after reflector poll.

Proposed resolution:

This wording is relative to N4892.

[Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]

Option A: Just fixes the most negative values

1. Modify 25.6.4.2 [range.iota.view] as indicated:

   constexpr auto size() const requires see below;
   

   -15- Effects: Equivalent to:

   if constexpr (is-integer-like<W> && is-integer-like<Bound>) {
     return (value_ < 0)
       ? ((bound_ < 0)
         ? to-unsigned-like(-value_) - to-unsigned-like(-bound_)
         : to-unsigned-like(bound_) + to-unsigned-like(-value_))
       : to-unsigned-like(bound_) - to-unsigned-like(value_);
     using UC = make-unsigned-like-t<common_type_t<W, Bound>>;
     return UC(bound_) - UC(value_);
   } else
     return to-unsigned-like(bound_ - value_);
   

   -16- Remarks: […]

Option B: Also fixes pathological cases involving unsigned-integer-like types

1. Modify 25.6.4.2 [range.iota.view] as indicated:

   constexpr auto size() const requires see below;
   

   -15- Effects: Equivalent to:

   if constexpr (is-integer-like<W> && is-integer-like<Bound>) {
     return (value_ < 0)
       ? ((bound_ < 0)
         ? to-unsigned-like(-value_) - to-unsigned-like(-bound_)
         : to-unsigned-like(bound_) + to-unsigned-like(-value_))
       : to-unsigned-like(bound_) - to-unsigned-like(value_);
     using UC = make-unsigned-like-t<common_type_t<W, Bound>>;
     if constexpr (is-signed-integer-like<W>)
       return UC(bound_) - UC(value_);
     else
       return UC(W(UC(bound_) - UC(value_)));
   } else
     return to-unsigned-like(bound_ - value_);
   

   -16- Remarks: […]