From 355ad75030bd94c91b378605d0bcf2e7da02c3c4 Mon Sep 17 00:00:00 2001
From: "Nevin \":-)\" Liber" <nliber@anl.gov>
Date: Mon, 22 Jul 2024 17:11:20 -0500
Subject: [PATCH] Added compile time test to show that arrays of different
 types or sizes are not equality comparable

---
 core/unit_test/TestArray.cpp | 59 ++++++++++++++++++++++++++++++++++++
 1 file changed, 59 insertions(+)

diff --git a/core/unit_test/TestArray.cpp b/core/unit_test/TestArray.cpp
index 62931e5a586..84698574dfd 100644
--- a/core/unit_test/TestArray.cpp
+++ b/core/unit_test/TestArray.cpp
@@ -15,6 +15,7 @@
 //@HEADER
 
 #include <Kokkos_Array.hpp>
+#include <Kokkos_DetectionIdiom.hpp>
 
 namespace {
 
@@ -23,6 +24,9 @@ namespace {
 template <typename... Ts>
 KOKKOS_FUNCTION constexpr void maybe_unused(Ts&&...) {}
 
+template <typename T, typename U = T>
+using equality_comparable = decltype(std::declval<T&>() == std::declval<U&>());
+
 KOKKOS_FUNCTION constexpr bool test_array() {
   constexpr Kokkos::Array<int, 3> a{{1, 2}};
 
@@ -182,4 +186,59 @@ constexpr bool test_to_array() {
 
 static_assert(test_to_array());
 
+constexpr bool test_array_equality_comparable() {
+  using C0 = Kokkos::Array<char, 0>;
+  using C2 = Kokkos::Array<char, 2>;
+  using C3 = Kokkos::Array<char, 3>;
+  using I0 = Kokkos::Array<int, 0>;
+  using I2 = Kokkos::Array<int, 2>;
+  using I3 = Kokkos::Array<int, 3>;
+
+  static_assert(Kokkos::is_detected_v<equality_comparable, C0, C0>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, C0, C2>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, C0, C3>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, C0, I0>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, C0, I2>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, C0, I3>);
+
+  static_assert(!Kokkos::is_detected_v<equality_comparable, C2, C0>);
+  static_assert(Kokkos::is_detected_v<equality_comparable, C2, C2>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, C2, C3>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, C2, I0>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, C2, I2>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, C2, I3>);
+
+  static_assert(!Kokkos::is_detected_v<equality_comparable, C3, C0>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, C3, C2>);
+  static_assert(Kokkos::is_detected_v<equality_comparable, C3, C3>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, C3, I0>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, C3, I2>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, C3, I3>);
+
+  static_assert(!Kokkos::is_detected_v<equality_comparable, I0, C0>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, I0, C2>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, I0, C3>);
+  static_assert(Kokkos::is_detected_v<equality_comparable, I0, I0>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, I0, I2>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, I0, I3>);
+
+  static_assert(!Kokkos::is_detected_v<equality_comparable, I2, C0>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, I2, C2>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, I2, C3>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, I2, I0>);
+  static_assert(Kokkos::is_detected_v<equality_comparable, I2, I2>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, I2, I3>);
+
+  static_assert(!Kokkos::is_detected_v<equality_comparable, I3, C0>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, I3, C2>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, I3, C3>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, I3, I0>);
+  static_assert(!Kokkos::is_detected_v<equality_comparable, I3, I2>);
+  static_assert(Kokkos::is_detected_v<equality_comparable, I3, I3>);
+
+  return true;
+}
+
+static_assert(test_array_equality_comparable());
+
 }  // namespace