[PATCH] libstdc++: Make sure iterator_traits is populated

[Patrick Palka]

Since basic_istream_view::iterator is neither a cpp17 iterator (because it's
move-only) nor does it define all four of the types {difference_type,
value_type, reference, iterator_category}, then by the rule in
[iterator.traits], its iterator_traits has no members.

More concretely this means that it can't e.g. be composed with a
views::transform or a views::filter because they look at the iterator_traits of
the underlying view.

This seems to be a defect in the current spec.

libstdc++-v3/ChangeLog:

        * include/std/ranges (ranges::basic_istream_view::iterator::reference):
        Define it so that this iterator's iterator_traits is populated.
        * testsuite/std/ranges/istream_view.cc: Augment test.
---
 libstdc++-v3/include/std/ranges                   |  1 +
 libstdc++-v3/testsuite/std/ranges/istream_view.cc | 11 +++++++++++
 2 files changed, 12 insertions(+)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 891ecf75eff..6e982d6ded3 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -996,6 +996,7 @@ namespace views
        using iterator_category = input_iterator_tag;
        using difference_type = ptrdiff_t;
        using value_type = _Val;
+       using reference = _Val&;
 
        _Iterator() = default;
 
diff --git a/libstdc++-v3/testsuite/std/ranges/istream_view.cc 
b/libstdc++-v3/testsuite/std/ranges/istream_view.cc
index 1729459bce3..7feb3ff4b0a 100644
--- a/libstdc++-v3/testsuite/std/ranges/istream_view.cc
+++ b/libstdc++-v3/testsuite/std/ranges/istream_view.cc
@@ -68,10 +68,21 @@ test03()
   VERIFY( ranges::equal(v, (int[]){0,1,2,3,4}) );
 }
 
+void
+test04()
+{
+  std::string s = "0123456789";
+  auto ss = std::istringstream{s};
+  static_assert(ranges::input_range<decltype(ranges::istream_view<X>(ss))>);
+  auto v = ranges::istream_view<X>(ss) | views::transform(&X::c);
+  VERIFY( ranges::equal(v, s) );
+}
+
 int
 main()
 {
   test01();
   test02();
   test03();
+  test04();
 }
-- 
2.25.0.191.gde93cc14ab