Implement the missing constexpr stuff in <array>. Fixes PR#31645.

llvm-svn: 292091
diff --git a/libcxx/test/std/containers/sequences/array/indexing.pass.cpp b/libcxx/test/std/containers/sequences/array/indexing.pass.cpp
index 64d2716..6cb7941 100644
--- a/libcxx/test/std/containers/sequences/array/indexing.pass.cpp
+++ b/libcxx/test/std/containers/sequences/array/indexing.pass.cpp
@@ -23,6 +23,14 @@
 // Disable the missing braces warning for this reason.
 #include "disable_missing_braces_warning.h"
 
+#if TEST_STD_VER > 14
+constexpr bool check_idx( size_t idx, double val )
+{ 
+    std::array<double, 3> arr = {1, 2, 3.5};
+	return arr[idx] == val;
+}
+#endif
+
 int main()
 {
     {
@@ -63,4 +71,11 @@
     }
 #endif
 
+#if TEST_STD_VER > 14
+    {
+        static_assert (check_idx(0, 1), "");
+        static_assert (check_idx(1, 2), "");
+        static_assert (check_idx(2, 3.5), "");
+    }
+#endif
 }