libstdc++: Fix noexcept-specifier for istream_iterator

Somehow I missed that the _M_value member can throw on construction.

	* include/bits/stream_iterator.h (istream_iterator(default_sentinel_t)):
	Make noexcept-specifier conditional.
	* testsuite/24_iterators/istream_iterator/cons/sentinel.cc: Check
	noexcept-specifier.

libstdc++-v3/ChangeLog

 2020-02-24  Jonathan Wakely  <jwakely@redhat.com>
 
 	* include/bits/stream_iterator.h (istream_iterator(default_sentinel_t)):
+	Make noexcept-specifier conditional.
+	* testsuite/24_iterators/istream_iterator/cons/sentinel.cc: Check
+	noexcept-specifier.
+
+	* include/bits/stream_iterator.h (istream_iterator(default_sentinel_t)):
 	Add constructor.
 	(operator==(istream_iterator, default_sentinel_t)): Add operator.
 	(ostream_iterator::difference_type): Define to ptrdiff_t for C++20.

libstdc++-v3/include/bits/stream_iterator.h

@@ -79,7 +79,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
 #if __cplusplus > 201703L
       constexpr
-      istream_iterator(default_sentinel_t) noexcept
+      istream_iterator(default_sentinel_t)
+      noexcept(is_nothrow_default_constructible_v<_Tp>)
       : istream_iterator() { }
 #endif
 

libstdc++-v3/testsuite/24_iterators/istream_iterator/cons/sentinel.cc

@@ -19,9 +19,18 @@
 // <http://www.gnu.org/licenses/>.
 
 #include <iterator>
+#include <istream>
 
 // C++20 doesn't require this to be non-throwing.
 static_assert( std::is_nothrow_constructible_v<std::istream_iterator<int>,
 					       std::default_sentinel_t> );
 
 constexpr std::istream_iterator<int> i = std::default_sentinel;
+
+struct X { X() noexcept(false); };
+std::istream& operator<<(std::istream&, X&);
+
+static_assert( std::is_constructible_v<std::istream_iterator<X>,
+				       std::default_sentinel_t> );
+static_assert( ! std::is_nothrow_constructible_v<std::istream_iterator<X>,
+						 std::default_sentinel_t> );