Add equality comparison

Author: rvarago

include/rvarago/refined.hpp

@@ -8,7 +8,10 @@ namespace rvarago::refined {
 
 // Properties a refinement implements.
 struct traits {
-  // Comparison operations with the spaceship operator.
+  // Equality comparison (`==`, `!=`).
+  bool equality{false};
+
+  // Comparison operations (`<`, `>`, etc) with the spaceship operator.
   bool ordered{false};
 };
 
@@ -64,6 +67,11 @@ class refinement {
     return refinement{std::move(value)};
   }
 
+  friend constexpr auto operator==(refinement const &, refinement const &)
+      -> bool
+    requires(Traits.equality)
+  = default;
+
   friend constexpr auto operator<=>(refinement const &, refinement const &)
     requires(Traits.ordered)
   = default;

test/refined_test.cpp

@@ -59,6 +59,15 @@ TEST_CASE("A to_exception policy should throw on invalid argument",
                     refined::error::to_exception::refinement_exception);
 }
 
+TEST_CASE("A refinement can be equality comparable", "[traits][equality]") {
+
+  using ref_cmp = refined::refinement<int, [](auto const &) { return true; },
+                                      refined::traits{.equality = true}>;
+
+  STATIC_REQUIRE(*ref_cmp::make(1) == *ref_cmp::make(1));
+  STATIC_REQUIRE(*ref_cmp::make(1) != *ref_cmp::make(2));
+}
+
 TEST_CASE("A refinement can be ordered", "[traits][sorted]") {
 
   using ref_cmp = refined::refinement<int, [](auto const &) { return true; },