constexpr_array: fix compile error in constexpr_array shim

LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) [year] [fullname]
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

include/ring-buffer-iterator.h

@@ -41,6 +41,10 @@ public:
         return {*container_ptr, front, index - n};
     }
 
+    CONSTEXPR difference_type operator-(const ring_buffer_iterator& other) const NOEXCEPT {
+        return index - other.index;
+    }
+
     CONSTEXPR reference operator*() COND_NOEXCEPT(noexcept(container_ptr->begin()) && noexcept(container_ptr->size())) {
         return *(container_ptr->begin() + (front + index) % container_ptr->size());
     }
@@ -90,7 +94,7 @@ public:
         return index >= other.index;
     }
 
-    CONSTEXPR friend ring_buffer_iterator operator+( difference_type n, const ring_buffer_iterator& it) NOEXCEPT {
+    CONSTEXPR friend ring_buffer_iterator operator+(difference_type n, const ring_buffer_iterator& it) NOEXCEPT {
         return it + n;
     }
     CONSTEXPR friend void swap(ring_buffer_iterator& a, ring_buffer_iterator& b) COND_NOEXCEPT(noexcept(std::swap(a.container, b.container))) {
@@ -137,6 +141,10 @@ public:
         return {*container_ptr, front, index - n};
     }
 
+    CONSTEXPR difference_type operator-(const ring_buffer_const_iterator& other) const NOEXCEPT {
+        return index - other.index;
+    }
+
     CONSTEXPR reference operator*() const COND_NOEXCEPT(noexcept(container_ptr->cbegin()) && noexcept(container_ptr->size())) {
         return *(container_ptr->cbegin() + (front + index) % container_ptr->size());
     }
@@ -186,7 +194,7 @@ public:
         return index >= other.index;
     }
 
-    CONSTEXPR friend ring_buffer_const_iterator operator+( difference_type n, const ring_buffer_const_iterator& it) NOEXCEPT {
+    CONSTEXPR friend ring_buffer_const_iterator operator+(difference_type n, const ring_buffer_const_iterator& it) NOEXCEPT {
         return it + n;
     }
     CONSTEXPR friend void swap(ring_buffer_const_iterator& a, ring_buffer_const_iterator& b) COND_NOEXCEPT(noexcept(std::swap(a.container, b.container))) {

test/constexpr-array-shim.h

@@ -5,7 +5,7 @@
 
 #if defined( __cpp_lib_array_constexpr) && __cpp_lib_array_constexpr >= 201606L
 template <typename T, size_t N>
-using constexpr_array = array;
+using constexpr_array = std::array<T, N>;
 #else
 template <typename T, size_t N>
 struct constexpr_array {