libsdc++: Restore check for validity of std::get for elements_view.

Resolves LWG3797, "elements_view insufficiently constrained".

When P2165R4 updated __has_tuple_element in C++23 to reuse __tuple_like
concept, it dropped the requirement of validity of get, assuming that for
tuple_like type with size of N, get<I> on lvalue is well-formed for any I < N.
This however does not hold for ranges::subrange (tuple-like of size 2) with
move-only iterator, for which get can only be applied on rvalue. In consequence
constrains allowed instantiating elements_view for range of such subrange,
but instantiating it's iterator lead to hard error from iterator_category
computation.

This patch applies the requirements on validity of get also in C++23 and
later standard modes.

libstdc++-v3/ChangeLog:

	* include/std/ranges (__detail::__has_tuple_element): Check
	if std::get<_Nm>(__t) returns referenceable type also for C++23
	and later.
	* testsuite/std/ranges/adaptors/elements.cc: Add test covering
	vector of ranges::subrange with move-only iterator.

Reviewed-by: Patrick Palka <ppalka@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 55d3c520ba4cc6d7e94009126702365e8cea3513..7d8b37f8c5cfbeca83b02c192de6c94fd4c6af05 100644 (file)
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -4355,8 +4355,15 @@ namespace views::__adaptor
   namespace __detail
   {
 #if __cpp_lib_tuple_like // >= C++23
+    // _GLIBCXX_RESOLVE_LIB_DEFECTS
+    // 3797. elements_view insufficiently constrained
     template<typename _Tp, size_t _Nm>
-    concept __has_tuple_element = __tuple_like<_Tp> && _Nm < tuple_size_v<_Tp>;
+    concept __has_tuple_element = __tuple_like<_Tp> && _Nm < tuple_size_v<_Tp>
+      && requires(_Tp __t)
+        {
+          { std::get<_Nm>(__t) }
+            -> convertible_to<const tuple_element_t<_Nm, _Tp>&>;
+        };
 #else
     template<typename _Tp, size_t _Nm>
     concept __has_tuple_element = requires(_Tp __t)

diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc
index 5237d787e7b21758933e8f58913c1af60c67f791..d384cfd5adf3be9a9dd5cd6131510155c28299c9 100644 (file)
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc
@@ -27,6 +27,22 @@
 namespace ranges = std::ranges;
 namespace views = ranges::views;
 
+template<size_t N, typename Rg>
+concept can_elements = requires (Rg&& rg)
+{ views::elements<0>(rg); };
+
+static_assert( !can_elements<0, std::vector<int>> );
+static_assert( can_elements<0, std::tuple<int>[4]> );
+static_assert( can_elements<1, std::vector<std::pair<int, int>>> );
+
+// Test LWG 3797. elements_view insufficiently constrained
+using move_only_iter_range = __gnu_test::test_input_range_nocopy<int>;
+using move_only_iter_subrange = ranges::subrange<
+  ranges::iterator_t<move_only_iter_range>,
+  ranges::sentinel_t<move_only_iter_range>>;
+static_assert( can_elements<0, std::vector<ranges::subrange<int*>>> );
+static_assert( !can_elements<0, std::vector<move_only_iter_subrange>> );
+
 void
 test01()
 {