types/description: add underlying_comparator for enum descriptions

test/types/description.cpp

@@ -5,7 +5,7 @@
 
 
 ///////////////////////////////////////////////////////////////////////////////
-enum an_enum_type { AN_ENUM_VALUE };
+enum an_enum_type : i16 { AN_ENUM_VALUE };
 
 
 //-----------------------------------------------------------------------------
@@ -55,6 +55,19 @@ int main ()
         "enums are described as such"
     );
 
+    {
+        auto const a = cruft::types::make_description<an_enum_type> ();
+        auto const b = cruft::types::make_description<std::underlying_type_t<an_enum_type>> ();
+
+        tap.expect_neq (a, b, "enum and underlying_type_t raw equality");
+
+        cruft::types::underlying_comparator const cmp {};
+        tap.expect (cmp (a, b), "enum and underlying_type equality");
+
+        auto const c = cruft::types::make_description<u32> ();
+        static_assert (!std::is_same_v<u32, std::underlying_type_t<an_enum_type>>);
+        tap.expect (!cmp (a, c), "enum and underlying_type inequality");
+    }
 
     auto const enum_type_descriptor = cruft::types::make_description<an_enum_type> ();
     tap.expect (

types/description.hpp

@@ -132,6 +132,45 @@ namespace cruft::types {
     }
 
 
+    /// A comparator that compares values as equal if they are an enum and the
+    /// underlying_type of said enum.
+    ///
+    /// This is distinct from the raw equality operator so as to not pessimise
+    /// the usability of the equality operator (for tasks that required strict
+    /// equality).
+    struct underlying_comparator {
+        bool operator() (
+            description const &a,
+            description const &b
+        ) const noexcept {
+            // If one of the descriptions is ENUM then both need to be either ENUM
+            // or INTEGER (because we try to compare enums and their underlying
+            // types as equal).
+            if (a.category == category::ENUM || b.category == category::ENUM) {
+                if (a.category != category::ENUM && a.category != category::INTEGER)
+                    return false;
+                if (b.category != category::ENUM && b.category != category::INTEGER)
+                    return false;
+
+                // Indices only need to match if they're both an ENUM,
+                // otherwise we're testing the underlying type.
+                if (a.category == category::ENUM && b.category == category::ENUM)
+                    if (a.index != b.index)
+                        return false;
+            } else {
+                if (a.index != b.index)
+                    return false;
+            }
+
+            // All details other must be identical;
+            return a.signedness == b.signedness &&
+                   a.width      == b.width      &&
+                   a.arity      == b.arity      &&
+                   a.alignment  == b.alignment;
+        };
+    };
+
+
     //-------------------------------------------------------------------------
     template <typename T, typename = void>
     struct category_traits;