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

[ericniebler]

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

constexpr iterator& operator--()
  requires ref_is_glvalue && BidirectionalRange<Base> &&
    BidirectionalRange<iter_reference_t<iterator_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 CommonRange, but nowhere is that requirement stated.

Proposed Resolution:

Change the synopsis of join_view::iterator in [range.join.iterator] as follows:

 constexpr iterator& operator--()
   requires ref_is_glvalue && BidirectionalRange<Base> &&
-    BidirectionalRange<iter_reference_t<iterator_t<Base>>>;
+    BidirectionalRange<iter_reference_t<iterator_t<Base>>> &&
+    CommonRange<iter_reference_t<iterator_t<Base>>>;
 
 constexpr iterator operator--(int)
   requires ref_is_glvalue && BidirectionalRange<Base> &&
-    BidirectionalRange<iter_reference_t<iterator_t<Base>>>;
+    BidirectionalRange<iter_reference_t<iterator_t<Base>>> &&
+    CommonRange<iter_reference_t<iterator_t<Base>>>;

Make a similar change to the declarations in [range.join.iterator]/14 and /15.

[CaseyCarter]

Edited the PR to end the requires-clause with ; instead of &&, and duplicated the requirements for operator--(int).