[libc++] views::split and views::lazy_split shouldn't be range adaptor closures (#75266)

Fixes #75002. Found while running libc++'s tests with MSVC's STL.

This is a superset of #74961 that also fixes the product code
and adds a regression test. Thanks again, @cpplearner!

To summarize: `views::split` and `views::lazy_split` aren't unary,
aren't range adaptor **closure** objects, and can't be piped. However,
\[range.adaptor.object\]/8 says that `views::split(pattern)` and
`views::lazy_split(pattern)` produce unary, pipeable, range adaptor
closure objects.

This PR adjusts the test coverage accordingly, allowing it to portably
pass for libc++ and MSVC's STL.

Author: StephanTLavavej

libcxx/include/__ranges/lazy_split_view.h

@@ -437,7 +437,7 @@ lazy_split_view(_Range&&, range_value_t<_Range>)
 
 namespace views {
 namespace __lazy_split_view {
-struct __fn : __range_adaptor_closure<__fn> {
+struct __fn {
   template <class _Range, class _Pattern>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
   constexpr auto operator()(_Range&& __range, _Pattern&& __pattern) const

libcxx/include/__ranges/split_view.h

@@ -194,7 +194,7 @@ struct split_view<_View, _Pattern>::__sentinel {
 
 namespace views {
 namespace __split_view {
-struct __fn : __range_adaptor_closure<__fn> {
+struct __fn {
   // clang-format off
   template <class _Range, class _Pattern>
   _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI

libcxx/test/std/ranges/range.adaptors/range.lazy.split/adaptor.pass.cpp

@@ -40,10 +40,16 @@ static_assert(!std::is_invocable_v<decltype(std::views::lazy_split), SomeView, N
 static_assert(!std::is_invocable_v<decltype(std::views::lazy_split), NotAView, SomeView>);
 static_assert( std::is_invocable_v<decltype(std::views::lazy_split), SomeView, SomeView>);
 
-static_assert( CanBePiped<SomeView&,    decltype(std::views::lazy_split)>);
-static_assert( CanBePiped<char(&)[10],  decltype(std::views::lazy_split)>);
-static_assert(!CanBePiped<char(&&)[10], decltype(std::views::lazy_split)>);
-static_assert(!CanBePiped<NotAView,     decltype(std::views::lazy_split)>);
+// Regression test for #75002, views::lazy_split shouldn't be a range adaptor closure
+static_assert(!CanBePiped<SomeView&, decltype(std::views::lazy_split)>);
+static_assert(!CanBePiped<char (&)[10], decltype(std::views::lazy_split)>);
+static_assert(!CanBePiped<char (&&)[10], decltype(std::views::lazy_split)>);
+static_assert(!CanBePiped<NotAView, decltype(std::views::lazy_split)>);
+
+static_assert(CanBePiped<SomeView&, decltype(std::views::lazy_split('x'))>);
+static_assert(CanBePiped<char (&)[10], decltype(std::views::lazy_split('x'))>);
+static_assert(!CanBePiped<char (&&)[10], decltype(std::views::lazy_split('x'))>);
+static_assert(!CanBePiped<NotAView, decltype(std::views::lazy_split('x'))>);
 
 static_assert(std::same_as<decltype(std::views::lazy_split), decltype(std::ranges::views::lazy_split)>);
 

libcxx/test/std/ranges/range.adaptors/range.split/adaptor.pass.cpp

@@ -39,10 +39,16 @@ static_assert(!std::is_invocable_v<decltype(std::views::split), SomeView, NotAVi
 static_assert(!std::is_invocable_v<decltype(std::views::split), NotAView, SomeView>);
 static_assert( std::is_invocable_v<decltype(std::views::split), SomeView, SomeView>);
 
-static_assert( CanBePiped<SomeView&,    decltype(std::views::split)>);
-static_assert( CanBePiped<char(&)[10],  decltype(std::views::split)>);
-static_assert(!CanBePiped<char(&&)[10], decltype(std::views::split)>);
-static_assert(!CanBePiped<NotAView,     decltype(std::views::split)>);
+// Regression test for #75002, views::split shouldn't be a range adaptor closure
+static_assert(!CanBePiped<SomeView&, decltype(std::views::split)>);
+static_assert(!CanBePiped<char (&)[10], decltype(std::views::split)>);
+static_assert(!CanBePiped<char (&&)[10], decltype(std::views::split)>);
+static_assert(!CanBePiped<NotAView, decltype(std::views::split)>);
+
+static_assert(CanBePiped<SomeView&, decltype(std::views::split('x'))>);
+static_assert(CanBePiped<char (&)[10], decltype(std::views::split('x'))>);
+static_assert(!CanBePiped<char (&&)[10], decltype(std::views::split('x'))>);
+static_assert(!CanBePiped<NotAView, decltype(std::views::split('x'))>);
 
 static_assert(std::same_as<decltype(std::views::split), decltype(std::ranges::views::split)>);