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.

2 files changed, 26 insertions, 23 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