[pushed] libstdc++: Make ranges::to closure objects SFINAE-friendly [PR112802]

[Patrick Palka]

This also happens to fix composition of these closure objects.

        PR libstdc++/112802
        PR libstdc++/113068

libstdc++-v3/ChangeLog:

        * include/std/ranges (__detail::_To::operator()): Add constraints.
        (__detail::_To2::operator()): Likewise.
        * testsuite/std/ranges/conv/1.cc (test_sfinae): New test.
        (test_composition): New test.
---
 libstdc++-v3/include/std/ranges             |  4 ++++
 libstdc++-v3/testsuite/std/ranges/conv/1.cc | 20 ++++++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index be8475c0cb1..752a04e01bd 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -9389,6 +9389,8 @@ namespace __detail
     struct _To
     {
       template<typename _Range, typename... _Args>
+       requires requires { ranges::to<_Cont>(std::declval<_Range>(),
+                                             std::declval<_Args>()...); }
        constexpr auto
        operator()(_Range&& __r, _Args&&... __args) const
        {
@@ -9431,6 +9433,8 @@ namespace __detail
     struct _To2
     {
       template<typename _Range, typename... _Args>
+       requires requires { ranges::to<_Cont>(std::declval<_Range>(),
+                                             std::declval<_Args>()...); }
        constexpr auto
        operator()(_Range&& __r, _Args&&... __args) const
        {
diff --git a/libstdc++-v3/testsuite/std/ranges/conv/1.cc 
b/libstdc++-v3/testsuite/std/ranges/conv/1.cc
index 6d6a708ab64..09fd515edf1 100644
--- a/libstdc++-v3/testsuite/std/ranges/conv/1.cc
+++ b/libstdc++-v3/testsuite/std/ranges/conv/1.cc
@@ -448,6 +448,24 @@ test_constexpr()
   static_assert(x == 6);
 }
 
+void
+test_sfinae()
+{
+  // PR libstdc++/112802
+  [](auto x) {
+    static_assert(!requires { std::ranges::to<std::vector<int>>()(x); });
+    static_assert(!requires { std::ranges::to<std::vector>()(x); });
+  }(0);
+}
+
+void
+test_composition()
+{
+  // PR libstdc++/113068
+  auto adaptor = std::ranges::to<std::string>() | 
std::ranges::to<std::string>();
+  auto str = adaptor(" ");
+}
+
 int main()
 {
   test_p1206r7_examples();
@@ -460,4 +478,6 @@ int main()
   test_lwg3984();
   test_nodiscard();
   test_constexpr();
+  test_sfinae();
+  test_composition();
 }
-- 
2.43.0.76.g1a87c842ec