add constexpr-array-shim to test constexpr on C++14

test/constexpr-array-shim.h

@@ -0,0 +1,56 @@
+#ifndef _CONSTEXPR_ARRAY_SHIM_H
+#define _CONSTEXPR_ARRAY_SHIM_H
+
+#include <array>
+
+#if defined( __cpp_lib_array_constexpr) && __cpp_lib_array_constexpr >= 201606L
+template <typename T, size_t N>
+using constexpr_array = array;
+#else
+template <typename T, size_t N>
+struct constexpr_array {
+    using size_type = size_t;
+    using value_type = T;
+
+    T data[N];
+
+    constexpr size_t size() const {
+        return N;
+    }
+
+    constexpr T* begin() {
+        return data;
+    }
+
+    constexpr T* end() {
+        return &data[N];
+    }
+
+    constexpr const T* begin() const {
+        return data;
+    }
+
+    constexpr const T* end() const {
+        return &data[N];
+    }
+
+    constexpr const T* cbegin() const {
+        return data;
+    }
+
+    constexpr const T* cend() const {
+        return &data[N];
+    }
+
+    constexpr T& operator[](size_t index) {
+        return data[index];
+    }
+
+    constexpr const T& operator[](size_t index) const {
+        return data[index];
+    }
+};
+
+#endif
+
+#endif