[gcc r14-6668] libstdc++: Make ranges::to closure objects SFINAE-friendly [PR112802]

[gcc r14-6668] libstdc++: Make ranges::to closure objects SFINAE-friendly [PR112802]

[Patrick Palka]

https://gcc.gnu.org/g:4f54e656788be3268dd45eb036447464a937fae9

commit r14-6668-g4f54e656788be3268dd45eb036447464a937fae9
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Dec 18 18:05:36 2023 -0500

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

Diff:
---
 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();
 }