libstdc++: Fix constraints for rvalue stream insertion/extraction

The __rval_streamable() function was an attempt to test for
convertibility cheaply and without confusing diagnostics. It doesn't
work with Clang though, and is probably ill-formed.

Replace that helper function with a check for derivation from ios_base,
and use that in the alias templates __rvalue_stream_insertion_t and
__rvalue_stream_extraction_t. Use concepts for the constraints when
available.

libstdc++-v3/ChangeLog:

	* include/std/istream (__rvalue_stream_extraction_t): Replace
	use of __rval_streamable.
	* include/std/ostream (__rvalue_stream_insertion_t): Likewise.
	(__rval_streamable): Remove.
	(_Require_derived_from_ios_base, __derived_from_ios_base): New
	helper for checking constraints.
	* testsuite/27_io/basic_istream/extractors_other/char/4.cc: Fix
	reference to the wrong subclause of the standard.
	* testsuite/27_io/basic_istream/extractors_other/wchar_t/4.cc:
	Likewise.
	* testsuite/27_io/basic_ostream/inserters_other/char/6.cc:
	Likewise.
	* testsuite/27_io/basic_ostream/inserters_other/wchar_t/6.cc:
	Likewise.
	* testsuite/27_io/basic_ostream/inserters_other/char/99692.cc:
	New test.
	* testsuite/27_io/filesystem/path/io/dr2989.cc: Adjust pruned
	errors.

8 files changed, 64 insertions, 28 deletions

diff --git a/libstdc++-v3/include/std/istream b/libstdc++-v3/include/std/istream
index ea34cce..5ad60db 100644
--- a/libstdc++-v3/include/std/istream
+++ b/libstdc++-v3/include/std/istream
@@ -958,12 +958,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // 2328. Rvalue stream extraction should use perfect forwarding
   // 1203. More useful rvalue stream insertion
 
-  // SFINAE helper to check constraints for operator>>(Istream&&, T&&).
-  // If the constraints are satisfied, it is an alias for Istream&&.
+#if __cpp_lib_concepts
+  template<typename _Is, typename _Tp>
+    requires __derived_from_ios_base<_Is>
+      && requires (_Is& __is, _Tp&& __t) { __is >> std::forward<_Tp>(__t); }
+    using __rvalue_stream_extraction_t = _Is&&;
+#else
   template<typename _Is, typename _Tp,
-	   typename = decltype(std::__rval_streamable<_Is>()
-				 >> std::declval<_Tp>())>
+	   typename = _Require_derived_from_ios_base<_Is>,
+	   typename = decltype(std::declval<_Is&>() >> std::declval<_Tp>())>
     using __rvalue_stream_extraction_t = _Is&&;
+#endif
 
   /**
    *  @brief  Generic extractor for rvalue stream
diff --git a/libstdc++-v3/include/std/ostream b/libstdc++-v3/include/std/ostream
index fdd2a87..9816973 100644
--- a/libstdc++-v3/include/std/ostream
+++ b/libstdc++-v3/include/std/ostream
@@ -708,31 +708,29 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // _GLIBCXX_RESOLVE_LIB_DEFECTS
   // 1203. More useful rvalue stream insertion
 
-  // SFINAE helper to check constraints for operator<<(Ostream&&, const T&).
-  // If Ostream is publicly and unambiguously derived from ios_base, then
-  // __rval_streamable<Ostream>() is equivalent to declval<Ostream&>().
-  // Otherwise, it results in a substitution failure. Specifically, it will
-  // fail if Ostream is an lvalue reference or the same type as ios_base.
-  // Use concepts if possible because they're cheaper to evaluate.
 #if __cpp_lib_concepts
+  // Use concepts if possible because they're cheaper to evaluate.
   template<typename _Tp>
-    requires (!is_same_v<_Tp, ios_base>)
-      && requires (_Tp* __t, ios_base* __b) { __b = __t; }
-    _Tp&
-    __rval_streamable();
+    concept __derived_from_ios_base = is_class_v<_Tp>
+      && (!is_same_v<_Tp, ios_base>)
+      && requires (_Tp* __t, ios_base* __b) { __b = __t; };
+
+  template<typename _Os, typename _Tp>
+    requires __derived_from_ios_base<_Os>
+      && requires (_Os& __os, const _Tp& __t) { __os << __t; }
+    using __rvalue_stream_insertion_t = _Os&&;
 #else
-  template<typename _Tp,
-	   typename = _Require<__not_<__is_one_of<_Tp, _Tp&, ios_base>>>>
-    _Tp&
-    __rval_streamable(ios_base* = (_Tp*)nullptr);
-#endif
+  template<typename _Tp>
+    using _Require_derived_from_ios_base
+      = _Require<is_class<_Tp>, __not_<is_same<_Tp, ios_base>>,
+		 is_convertible<typename add_pointer<_Tp>::type, ios_base*>>;
 
-  // SFINAE helper to check constraints for operator<<(Ostream&&, const T&).
-  // If the constraints are satisfied, it is an alias for Ostream&&.
   template<typename _Os, typename _Tp,
-	   typename = decltype(std::__rval_streamable<_Os>()
-				 << std::declval<const _Tp&>())>
+	   typename = _Require_derived_from_ios_base<_Os>,
+	   typename
+	     = decltype(std::declval<_Os&>() << std::declval<const _Tp&>())>
     using __rvalue_stream_insertion_t = _Os&&;
+#endif
 
   /**
    *  @brief  Generic inserter for rvalue stream
diff --git a/libstdc++-v3/testsuite/27_io/basic_istream/extractors_other/char/4.cc b/libstdc++-v3/testsuite/27_io/basic_istream/extractors_other/char/4.cc
index 9f1e293..94e41a1 100644
--- a/libstdc++-v3/testsuite/27_io/basic_istream/extractors_other/char/4.cc
+++ b/libstdc++-v3/testsuite/27_io/basic_istream/extractors_other/char/4.cc
@@ -17,7 +17,7 @@
 // with this library; see the file COPYING3.  If not see
 // <http://www.gnu.org/licenses/>.
 
-// 27.6.2.5.3 basic_ostream manipulator inserters
+// C++11 27.7.2.6 Rvalue stream extraction [istream.rvalue]
 
 #include <sstream>
 
diff --git a/libstdc++-v3/testsuite/27_io/basic_istream/extractors_other/wchar_t/4.cc b/libstdc++-v3/testsuite/27_io/basic_istream/extractors_other/wchar_t/4.cc
index fc7f550..b182be7 100644
--- a/libstdc++-v3/testsuite/27_io/basic_istream/extractors_other/wchar_t/4.cc
+++ b/libstdc++-v3/testsuite/27_io/basic_istream/extractors_other/wchar_t/4.cc
@@ -17,7 +17,7 @@
 // with this library; see the file COPYING3.  If not see
 // <http://www.gnu.org/licenses/>.
 
-// 27.6.2.5.3 basic_ostream manipulator inserters
+// C++11 27.7.2.6 Rvalue stream extraction [istream.rvalue]
 
 #include <sstream>
 
diff --git a/libstdc++-v3/testsuite/27_io/basic_ostream/inserters_other/char/6.cc b/libstdc++-v3/testsuite/27_io/basic_ostream/inserters_other/char/6.cc
index 4801afc..3a748cd 100644
--- a/libstdc++-v3/testsuite/27_io/basic_ostream/inserters_other/char/6.cc
+++ b/libstdc++-v3/testsuite/27_io/basic_ostream/inserters_other/char/6.cc
@@ -17,7 +17,7 @@
 // with this library; see the file COPYING3.  If not see
 // <http://www.gnu.org/licenses/>.
 
-// 27.6.2.5.3 basic_ostream manipulator inserters
+// C++11 27.7.3.9 Rvalue stream insertion [ostream.rvalue]
 
 #include <sstream>
 
diff --git a/libstdc++-v3/testsuite/27_io/basic_ostream/inserters_other/char/99692.cc b/libstdc++-v3/testsuite/27_io/basic_ostream/inserters_other/char/99692.cc
new file mode 100644
index 0000000..e41399a
--- /dev/null
+++ b/libstdc++-v3/testsuite/27_io/basic_ostream/inserters_other/char/99692.cc
@@ -0,0 +1,34 @@
+// { dg-do compile { target c++11 } }
+
+#include <ostream>
+
+struct CustomStream : std::ostream {};
+
+namespace N {
+    class A{};
+}
+
+std::ostream& operator<<(std::ostream& s, const N::A&)
+{
+    return s;
+}
+
+CustomStream&& operator<<(CustomStream&& s, const N::A& v)
+{
+    static_cast<std::ostream&>(s) << v;
+    return std::move(s);
+}
+
+void test_pr99692()
+{
+  // PR libstdc++/99692
+    CustomStream() << N::A{};
+}
+
+int test_shift_ios_enum()
+{
+  // https://gcc.gnu.org/pipermail/libstdc++/2021-May/052507.html
+  int i = 1 << std::ios::erase_event;
+
+  return i;
+}
diff --git a/libstdc++-v3/testsuite/27_io/basic_ostream/inserters_other/wchar_t/6.cc b/libstdc++-v3/testsuite/27_io/basic_ostream/inserters_other/wchar_t/6.cc
index 3efeb80..8fc0694 100644
--- a/libstdc++-v3/testsuite/27_io/basic_ostream/inserters_other/wchar_t/6.cc
+++ b/libstdc++-v3/testsuite/27_io/basic_ostream/inserters_other/wchar_t/6.cc
@@ -17,7 +17,7 @@
 // with this library; see the file COPYING3.  If not see
 // <http://www.gnu.org/licenses/>.
 
-// 27.6.2.5.3 basic_ostream manipulator inserters
+// C++11 27.7.3.9 Rvalue stream insertion [ostream.rvalue]
 
 #include <sstream>
 
diff --git a/libstdc++-v3/testsuite/27_io/filesystem/path/io/dr2989.cc b/libstdc++-v3/testsuite/27_io/filesystem/path/io/dr2989.cc
index 609f1c3..c5cda77 100644
--- a/libstdc++-v3/testsuite/27_io/filesystem/path/io/dr2989.cc
+++ b/libstdc++-v3/testsuite/27_io/filesystem/path/io/dr2989.cc
@@ -33,4 +33,3 @@ void foo(std::iostream& s) {
   s >> p; // { dg-error "no match" }
 }
 // { dg-prune-output "no type .*enable_if" }
-// { dg-prune-output "no matching function for call to '__rval_streamable" }