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3313. join_view::iterator::operator-- is incorrectly constrained

Section: 26.7.14.3 [range.join.iterator] Status: C++20 Submitter: United States Opened: 2019-11-04 Last modified: 2021-02-25

Priority: 0

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Discussion:

Addresses US 294

join_view::iterator::operator-- is improperly constrained. In the Effects: clause in paragraph 14, we see the statement:

inner_ = ranges::end(*--outer_);

However, this only well-formed when end returns an iterator, not a sentinel. This requirement is not reflected in the constraints of the function(s).

Eric Niebler:

From the WD, join_view::iterator::operator-- is specified as:

constexpr iterator& operator--()
  requires ref_is_glvalue && bidirectional_range<Base> &&
    bidirectional_range<range_reference_t<Base>>;

-14- Effects: Equivalent to:

if (outer_ == ranges::end(parent_->base_))
  inner_ = ranges::end(*--outer_);
while (inner_ == ranges::begin(*outer_))
  inner_ = ranges::end(*--outer_);
--inner_;
return *this;

The trouble is from the lines that do:

  inner_ = ranges::end(*--outer_);

Clearly this will only compile when *--outer returns a common_range, but nowhere is that requirement stated.

[2019-11 Status to Ready during Tuesday morning issue processing in Belfast.]

Proposed resolution:

This wording is relative to N4835.

1. Modify 26.7.14.3 [range.join.iterator], class template join_view::iterator synopsis, as indicated:

   constexpr iterator& operator--()
     requires ref_is_glvalue && bidirectional_range<Base> &&
              bidirectional_range<range_reference_t<Base>> &&
              common_range<range_reference_t<Base>>;
   
   constexpr iterator operator--(int)
     requires ref_is_glvalue && bidirectional_range<Base> &&
              bidirectional_range<range_reference_t<Base>> &&
              common_range<range_reference_t<Base>>;
   

2. Modify 26.7.14.3 [range.join.iterator] as indicated:

   constexpr iterator& operator--()
     requires ref_is_glvalue && bidirectional_range<Base> &&
              bidirectional_range<range_reference_t<Base>> &&
              common_range<range_reference_t<Base>>;
   

   -14- Effects: Equivalent to:

   if (outer_ == ranges::end(parent_->base_))
     inner_ = ranges::end(*--outer_);
   while (inner_ == ranges::begin(*outer_))
     inner_ = ranges::end(*--outer_);
   --inner_;
   return *this;
   

   constexpr iterator operator--(int)
     requires ref_is_glvalue && bidirectional_range<Base> &&
              bidirectional_range<range_reference_t<Base>> &&
              common_range<range_reference_t<Base>>;
   

   -15- Effects: Equivalent to:

   auto tmp = *this;
   --*this;
   return tmp;