libstdc++: Change __cpp_lib_array_constexpr for C++17 again

This partially reverts my previous change related to this macro. The
C++20 constexpr iterator requirements are always met by array:iterator,
because it's just a pointer. So the macro can be set to 201803 even in
C++17 mode.

	* include/bits/stl_iterator.h (__cpp_lib_array_constexpr): Revert
	value for C++17 to 201803L because P0858R0 is supported for C++17.
	* include/std/version (__cpp_lib_array_constexpr): Likewise.
	* testsuite/23_containers/array/element_access/constexpr_c++17.cc:
	Check for value corresponding to P0031R0 features being tested.
	* testsuite/23_containers/array/requirements/constexpr_iter.cc:
	Check for value corresponding to P0858R0 features being tested.

libstdc++-v3/ChangeLog

 2020-04-23  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/bits/stl_iterator.h (__cpp_lib_array_constexpr): Revert
+	value for C++17 to 201803L because P0858R0 is supported for C++17.
+	* include/std/version (__cpp_lib_array_constexpr): Likewise.
+	* testsuite/23_containers/array/element_access/constexpr_c++17.cc:
+	Check for value corresponding to P0031R0 features being tested.
+	* testsuite/23_containers/array/requirements/constexpr_iter.cc:
+	Check for value corresponding to P0858R0 features being tested.
+
 	* include/std/version (__cpp_lib_three_way_comparison): Define for
 	freestanding builds.
 

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

@@ -72,7 +72,7 @@
 #if __cplusplus > 201703L
 # define __cpp_lib_array_constexpr 201811L
 #elif __cplusplus == 201703L
-# define __cpp_lib_array_constexpr 201603L
+# define __cpp_lib_array_constexpr 201803L
 #endif
 
 #if __cplusplus > 201703L

libstdc++-v3/include/std/version

@@ -123,7 +123,7 @@
 #if _GLIBCXX_HOSTED
 #define __cpp_lib_any 201606L
 #define __cpp_lib_apply 201603
-#define __cpp_lib_array_constexpr 201603L
+#define __cpp_lib_array_constexpr 201803L
 #define __cpp_lib_as_const 201510
 #define __cpp_lib_boyer_moore_searcher 201603
 #define __cpp_lib_chrono 201611

libstdc++-v3/testsuite/23_containers/array/element_access/constexpr_c++17.cc

@@ -24,8 +24,6 @@
 # error "Feature test macro for array constexpr is missing in <array>"
 #elif __cpp_lib_array_constexpr < 201603L
 # error "Feature test macro for array constexpr has wrong value in <array>"
-#elif __cpp_lib_array_constexpr > 201603L && __cplusplus == 201703
-# error "Feature test macro for array constexpr has wrong value for C++17"
 #endif
 
 constexpr std::size_t test01()

libstdc++-v3/testsuite/23_containers/array/requirements/constexpr_iter.cc

@@ -22,12 +22,13 @@
 
 #ifndef __cpp_lib_array_constexpr
 # error "Feature test macro for array constexpr is missing in <array>"
-#elif __cpp_lib_array_constexpr < 201603L
+#elif __cpp_lib_array_constexpr < 201803L
 # error "Feature test macro for array constexpr has wrong value in <array>"
-#elif __cpp_lib_array_constexpr > 201603L && __cplusplus == 201703
-# error "Feature test macro for array constexpr has wrong value for C++17"
 #endif
 
+// This test is compiled as C++17 because array::iterator is just a pointer,
+// so always meets the C++20 constexpr iterator requirements, even in C++17.
+
 constexpr int
 test()
 {