libstdc++: Implement LWG 1203 for rvalue iostreams

This implements the resolution of LWG 1203 so that the constraints for
rvalue stream insertion/extraction are simpler, and the return type is
the original rvalue stream type not its base class.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* include/std/istream (operator>>(Istream&&, x&)): Simplify, as
	per LWG 1203.
	* include/std/ostream (operator<<(Ostream&&, const x&)):
	Likewise.
	* testsuite/27_io/basic_istream/extractors_character/char/lwg2499_neg.cc:
	Adjust dg-error pattern.
	* testsuite/27_io/basic_istream/extractors_character/wchar_t/lwg2499_neg.cc:
	Likewise.
	* testsuite/27_io/basic_istream/extractors_other/char/4.cc: Define
	is_extractable trait to replace std::__is_extractable. Make it
	work with rvalue streams as well as lvalues, to replace f() and
	g() helper functions.
	* testsuite/27_io/basic_istream/extractors_other/wchar_t/4.cc:
	Likewise.
	* testsuite/27_io/basic_ostream/inserters_other/char/6.cc:
	Define is_insertable trait to replace std::__is_insertable. Make
	it work with rvalue streams as well as lvalues, to replace f()
	and g() helper functions.
	* testsuite/27_io/basic_ostream/inserters_other/wchar_t/6.cc:
	Likewise.
	* testsuite/27_io/filesystem/path/io/dr2989.cc: Prune additional
	errors from new constraints.
	* testsuite/27_io/rvalue_streams-2.cc: Remove PR 80675 checks,
	which are no longer expected to compile.
	* testsuite/27_io/rvalue_streams.cc: Adjust existing test.
	Verify LWG 1203 changes.

4 files changed, 62 insertions, 108 deletions

diff --git a/libstdc++-v3/testsuite/27_io/basic_istream/extractors_character/char/lwg2499_neg.cc b/libstdc++-v3/testsuite/27_io/basic_istream/extractors_character/char/lwg2499_neg.cc
index 7a39e7c..abd68af 100644
--- a/libstdc++-v3/testsuite/27_io/basic_istream/extractors_character/char/lwg2499_neg.cc
+++ b/libstdc++-v3/testsuite/27_io/basic_istream/extractors_character/char/lwg2499_neg.cc
@@ -26,9 +26,9 @@
 void
 test01(std::istream& in, char* pc, signed char* sc, unsigned char* uc)
 {
-  in >> pc; // { dg-error "here" }
-  in >> sc; // { dg-error "here" }
-  in >> uc; // { dg-error "here" }
+  in >> pc; // { dg-error "no match" }
+  in >> sc; // { dg-error "no match" }
+  in >> uc; // { dg-error "no match" }
 }
 
 struct CT : std::char_traits<char> { };
@@ -37,9 +37,9 @@ void
 test02(std::basic_istream<char, CT>& in, char* pc, signed char* sc,
        unsigned char* uc)
 {
-  in >> pc; // { dg-error "here" }
-  in >> sc; // { dg-error "here" }
-  in >> uc; // { dg-error "here" }
+  in >> pc; // { dg-error "no match" }
+  in >> sc; // { dg-error "no match" }
+  in >> uc; // { dg-error "no match" }
 }
 
 // { dg-excess-errors "" }
diff --git a/libstdc++-v3/testsuite/27_io/basic_istream/extractors_character/wchar_t/lwg2499_neg.cc b/libstdc++-v3/testsuite/27_io/basic_istream/extractors_character/wchar_t/lwg2499_neg.cc
index bc8dfac..214f8d6 100644
--- a/libstdc++-v3/testsuite/27_io/basic_istream/extractors_character/wchar_t/lwg2499_neg.cc
+++ b/libstdc++-v3/testsuite/27_io/basic_istream/extractors_character/wchar_t/lwg2499_neg.cc
@@ -26,7 +26,7 @@
 void
 test01(std::wistream& in, wchar_t* wc)
 {
-  in >> wc; // { dg-error "here" }
+  in >> wc; // { dg-error "no match" }
 }
 
 struct WT : std::char_traits<wchar_t> { };
@@ -34,7 +34,7 @@ struct WT : std::char_traits<wchar_t> { };
 void
 test02(std::basic_istream<wchar_t, WT>& in, wchar_t* wc)
 {
-  in >> wc; // { dg-error "here" }
+  in >> wc; // { dg-error "no match" }
 }
 
 // { dg-excess-errors "" }
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 5ad1c1b..9f1e293 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
@@ -21,6 +21,21 @@
 
 #include <sstream>
 
+template<typename Istream, typename T, typename = void>
+  struct is_extractable : std::false_type
+  { };
+
+template<typename> using void_t = void;
+
+template<typename Istream, typename T>
+  using extract_result
+    = decltype(std::declval<Istream>() >> std::declval<const T&>());
+
+template<typename Istream, typename T>
+  struct is_extractable<Istream, T, void_t<extract_result<Istream, T>>>
+  : std::true_type
+  { };
+
 struct X {};
 std::istream& operator>>(std::istream&, X&) = delete;
 
@@ -30,20 +45,6 @@ std::istream& operator>>(std::istream& is, Y&&) {return is;}
 
 struct Z{};
 
-template <class T>
-auto f(T&&) -> decltype(void(std::declval<std::istream&>()
-			     >> std::declval<T&&>()),
-			std::true_type());
-
-std::false_type f(...);
-
-template <class T>
-auto g(T&&) -> decltype(void(std::declval<std::istream&&>()
-			     >> std::declval<T&&>()),
-			std::true_type());
-
-std::false_type g(...);
-
 void test01()
 {
   Y y;
@@ -52,42 +53,18 @@ void test01()
   is >> Y();
   std::istringstream() >> y;
   std::istringstream() >> Y();
-  static_assert(!std::__is_extractable<std::istream&, X&>::value, "");
-  static_assert(!std::__is_extractable<std::istream&&, X&>::value, "");
-  static_assert(!std::__is_extractable<std::istream&, X&&>::value, "");
-  static_assert(!std::__is_extractable<std::istream&&, X&&>::value, "");
-  static_assert(std::__is_extractable<std::istream&, Y&>::value, "");
-  static_assert(std::__is_extractable<std::istream&&, Y&>::value, "");
-  static_assert(std::__is_extractable<std::istream&, Y&&>::value, "");
-  static_assert(std::__is_extractable<std::istream&&, Y&&>::value, "");
-  static_assert(!std::__is_extractable<std::istream&, Z&>::value, "");
-  static_assert(!std::__is_extractable<std::istream&&, Z&>::value, "");
-  static_assert(!std::__is_extractable<std::istream&, Z&&>::value, "");
-  static_assert(!std::__is_extractable<std::istream&&, Z&&>::value, "");
-  static_assert(std::is_same<decltype(f(std::declval<X&>())),
-		std::false_type>::value, "");
-  static_assert(std::is_same<decltype(f(std::declval<X&&>())),
-		std::false_type>::value, "");
-  static_assert(std::is_same<decltype(f(std::declval<Y&>())),
-		std::true_type>::value, "");
-  static_assert(std::is_same<decltype(f(std::declval<Y&&>())),
-		std::true_type>::value, "");
-  static_assert(std::is_same<decltype(f(std::declval<Z&>())),
-		std::false_type>::value, "");
-  static_assert(std::is_same<decltype(f(std::declval<Z&&>())),
-		std::false_type>::value, "");
-  static_assert(std::is_same<decltype(g(std::declval<X&>())),
-		std::false_type>::value, "");
-  static_assert(std::is_same<decltype(g(std::declval<X&&>())),
-		std::false_type>::value, "");
-  static_assert(std::is_same<decltype(g(std::declval<Y&>())),
-		std::true_type>::value, "");
-  static_assert(std::is_same<decltype(g(std::declval<Y&&>())),
-		std::true_type>::value, "");
-  static_assert(std::is_same<decltype(g(std::declval<Z&>())),
-		std::false_type>::value, "");
-  static_assert(std::is_same<decltype(g(std::declval<Z&&>())),
-		std::false_type>::value, "");
+  static_assert(!is_extractable<std::istream&, X&>::value, "");
+  static_assert(!is_extractable<std::istream&&, X&>::value, "");
+  static_assert(!is_extractable<std::istream&, X&&>::value, "");
+  static_assert(!is_extractable<std::istream&&, X&&>::value, "");
+  static_assert(is_extractable<std::istream&, Y&>::value, "");
+  static_assert(is_extractable<std::istream&&, Y&>::value, "");
+  static_assert(is_extractable<std::istream&, Y&&>::value, "");
+  static_assert(is_extractable<std::istream&&, Y&&>::value, "");
+  static_assert(!is_extractable<std::istream&, Z&>::value, "");
+  static_assert(!is_extractable<std::istream&&, Z&>::value, "");
+  static_assert(!is_extractable<std::istream&, Z&&>::value, "");
+  static_assert(!is_extractable<std::istream&&, Z&&>::value, "");
 }
 
 int main()
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 94f2d9d..fc7f550 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
@@ -21,6 +21,21 @@
 
 #include <sstream>
 
+template<typename Istream, typename T, typename = void>
+  struct is_extractable : std::false_type
+  { };
+
+template<typename> using void_t = void;
+
+template<typename Istream, typename T>
+  using extract_result
+    = decltype(std::declval<Istream>() >> std::declval<const T&>());
+
+template<typename Istream, typename T>
+  struct is_extractable<Istream, T, void_t<extract_result<Istream, T>>>
+  : std::true_type
+  { };
+
 struct X {};
 std::wistream& operator>>(std::wistream&, X&) = delete;
 
@@ -30,20 +45,6 @@ std::wistream& operator>>(std::wistream& is, Y&&) {return is;}
 
 struct Z{};
 
-template <class T>
-auto f(T&&) -> decltype(void(std::declval<std::wistream&>()
-			     >> std::declval<T&&>()),
-			std::true_type());
-
-std::false_type f(...);
-
-template <class T>
-auto g(T&&) -> decltype(void(std::declval<std::wistream&&>()
-			     >> std::declval<T&&>()),
-			std::true_type());
-
-std::false_type g(...);
-
 void test01()
 {
   Y y;
@@ -52,42 +53,18 @@ void test01()
   is >> Y();
   std::wistringstream() >> y;
   std::wistringstream() >> Y();
-  static_assert(!std::__is_extractable<std::wistream&, X&>::value, "");
-  static_assert(!std::__is_extractable<std::wistream&&, X&>::value, "");
-  static_assert(!std::__is_extractable<std::wistream&, X&&>::value, "");
-  static_assert(!std::__is_extractable<std::wistream&&, X&&>::value, "");
-  static_assert(std::__is_extractable<std::wistream&, Y&>::value, "");
-  static_assert(std::__is_extractable<std::wistream&&, Y&>::value, "");
-  static_assert(std::__is_extractable<std::wistream&, Y&&>::value, "");
-  static_assert(std::__is_extractable<std::wistream&&, Y&&>::value, "");
-  static_assert(!std::__is_extractable<std::wistream&, Z&>::value, "");
-  static_assert(!std::__is_extractable<std::wistream&&, Z&>::value, "");
-  static_assert(!std::__is_extractable<std::wistream&, Z&&>::value, "");
-  static_assert(!std::__is_extractable<std::wistream&&, Z&&>::value, "");
-  static_assert(std::is_same<decltype(f(std::declval<X&>())),
-		std::false_type>::value, "");
-  static_assert(std::is_same<decltype(f(std::declval<X&&>())),
-		std::false_type>::value, "");
-  static_assert(std::is_same<decltype(f(std::declval<Y&>())),
-		std::true_type>::value, "");
-  static_assert(std::is_same<decltype(f(std::declval<Y&&>())),
-		std::true_type>::value, "");
-  static_assert(std::is_same<decltype(f(std::declval<Z&>())),
-		std::false_type>::value, "");
-  static_assert(std::is_same<decltype(f(std::declval<Z&&>())),
-		std::false_type>::value, "");
-  static_assert(std::is_same<decltype(g(std::declval<X&>())),
-		std::false_type>::value, "");
-  static_assert(std::is_same<decltype(g(std::declval<X&&>())),
-		std::false_type>::value, "");
-  static_assert(std::is_same<decltype(g(std::declval<Y&>())),
-		std::true_type>::value, "");
-  static_assert(std::is_same<decltype(g(std::declval<Y&&>())),
-		std::true_type>::value, "");
-  static_assert(std::is_same<decltype(g(std::declval<Z&>())),
-		std::false_type>::value, "");
-  static_assert(std::is_same<decltype(g(std::declval<Z&&>())),
-		std::false_type>::value, "");
+  static_assert(!is_extractable<std::wistream&, X&>::value, "");
+  static_assert(!is_extractable<std::wistream&&, X&>::value, "");
+  static_assert(!is_extractable<std::wistream&, X&&>::value, "");
+  static_assert(!is_extractable<std::wistream&&, X&&>::value, "");
+  static_assert(is_extractable<std::wistream&, Y&>::value, "");
+  static_assert(is_extractable<std::wistream&&, Y&>::value, "");
+  static_assert(is_extractable<std::wistream&, Y&&>::value, "");
+  static_assert(is_extractable<std::wistream&&, Y&&>::value, "");
+  static_assert(!is_extractable<std::wistream&, Z&>::value, "");
+  static_assert(!is_extractable<std::wistream&&, Z&>::value, "");
+  static_assert(!is_extractable<std::wistream&, Z&&>::value, "");
+  static_assert(!is_extractable<std::wistream&&, Z&&>::value, "");
 }
 
 int main()