libstdc++: Fix compilation with released versions of Clang

Clang 9 supports C++20 via -std=c++2a but doesn't support Concepts, so
several of the new additions related to the Ranges library fail to
compile with -std=c++2a. The new definition of iterator_traits and the
definition of default_sentinel_t are guarded by __cpp_lib_concepts, so
check that in addition to __cplusplus > 201703L.

	* include/bits/stl_algobase.h (__lexicographical_compare_aux): Check
	__cpp_lib_concepts before using iter_reference_t.
	* include/bits/stream_iterator.h (istream_iterator): Check
	__cpp_lib_concepts before using default_sentinel_t.
	* include/bits/streambuf_iterator.h (istreambuf_iterator): Likewise.

libstdc++-v3/ChangeLog

 2020-03-18  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/bits/stl_algobase.h (__lexicographical_compare_aux): Check
+	__cpp_lib_concepts before using iter_reference_t.
+	* include/bits/stream_iterator.h (istream_iterator): Check
+	__cpp_lib_concepts before using default_sentinel_t.
+	* include/bits/streambuf_iterator.h (istreambuf_iterator): Likewise.
+
 	PR libstdc++/94203
 	* include/experimental/executor (executor::executor(Executor)): Call
 	make_shared directly instead of _M_create. Create _Tgt1 object.

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

@@ -1283,7 +1283,7 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
 	 && !__gnu_cxx::__numeric_traits<_ValueType2>::__is_signed
 	 && __is_pointer<_II1>::__value
 	 && __is_pointer<_II2>::__value
-#if __cplusplus > 201703L
+#if __cplusplus > 201703L && __cpp_lib_concepts
 	 // For C++20 iterator_traits<volatile T*>::value_type is non-volatile
 	 // so __is_byte<T> could be true, but we can't use memcmp with
 	 // volatile data.

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

@@ -77,7 +77,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
         _M_ok(__obj._M_ok)
       { }
 
-#if __cplusplus > 201703L
+#if __cplusplus > 201703L && __cpp_lib_concepts
       constexpr
       istream_iterator(default_sentinel_t)
       noexcept(is_nothrow_default_constructible_v<_Tp>)
@@ -153,7 +153,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       operator!=(const istream_iterator& __x, const istream_iterator& __y)
       { return !__x._M_equal(__y); }
 
-#if __cplusplus > 201703L
+#if __cplusplus > 201703L && __cpp_lib_concepts
       friend bool
       operator==(const istream_iterator& __i, default_sentinel_t)
       { return !__i._M_stream; }

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

@@ -115,7 +115,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       _GLIBCXX_CONSTEXPR istreambuf_iterator() _GLIBCXX_USE_NOEXCEPT
       : _M_sbuf(0), _M_c(traits_type::eof()) { }
 
-#if __cplusplus > 201703L
+#if __cplusplus > 201703L && __cpp_lib_concepts
       constexpr istreambuf_iterator(default_sentinel_t) noexcept
       : istreambuf_iterator() { }
 #endif
@@ -215,7 +215,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return traits_type::eq_int_type(__c, __eof);
       }
 
-#if __cplusplus > 201703L
+#if __cplusplus > 201703L && __cpp_lib_concepts
       friend bool
       operator==(const istreambuf_iterator& __i, default_sentinel_t __s)
       { return __i._M_at_eof(); }