@@ -10,6 +10,7 @@ This document follows the conventions laid out in [Keep a CHANGELOG][].
 ### Added
 
 -   Extension methods declared in non-framework assemblies are bound to the type they extend, so they can be called from the instance like in C# (e.g. `order_status.is_open()`). Those marked `[EditorBrowsable(EditorBrowsableState.Never)]`, or declared in a class marked so, keep their static form only
+-   Types with an implicit conversion to a numeric type support Python arithmetic, comparisons, `float()` and `int()` through that conversion, behaving like a Python `int` for integral conversions and a `float` otherwise, unless they declare or inherit the operator
 -   Ability to instantiate new .NET arrays using `Array[T](dim1, dim2, ...)` syntax
 -   Python operator method will call C# operator method for supported binary and unary operators ([#1324][p1324]).
 -   Add GetPythonThreadID and Interrupt methods in PythonEngine
 
@@ -0,0 +1,367 @@
+using System;
+using NUnit.Framework;
+using Python.Runtime;
+
+namespace Python.EmbeddingTest
+{
+    public class ConvertibleFee
+    {
+        public decimal Amount { get; }
+
+        public ConvertibleFee(decimal amount)
+        {
+            Amount = amount;
+        }
+
+        public static implicit operator decimal(ConvertibleFee fee) => fee.Amount;
+    }
+
+    public class ConvertibleRatio
+    {
+        public double Value { get; }
+
+        public ConvertibleRatio(double value)
+        {
+            Value = value;
+        }
+
+        public static implicit operator double(ConvertibleRatio ratio) => ratio.Value;
+    }
+
+    public class ConvertibleMoney
+    {
+        public decimal Amount { get; }
+
+        public ConvertibleMoney(decimal amount)
+        {
+            Amount = amount;
+        }
+
+        public static implicit operator decimal(ConvertibleMoney money) => money.Amount;
+
+        // the type's own operator must keep winning over the synthesized one
+        public static ConvertibleMoney operator +(ConvertibleMoney a, ConvertibleMoney b) => new(a.Amount + b.Amount);
+    }
+
+    public class ConvertibleCount
+    {
+        public long Value { get; }
+
+        public ConvertibleCount(long value)
+        {
+            Value = value;
+        }
+
+        public static implicit operator long(ConvertibleCount count) => count.Value;
+    }
+
+    public class ConvertibleBase
+    {
+        // inherited by ConvertibleDerived, it must keep winning over the synthesized one as in C#
+        public static string operator +(ConvertibleBase a, ConvertibleBase b) => "base";
+    }
+
+    public class ConvertibleDerived : ConvertibleBase
+    {
+        public decimal Amount { get; }
+
+        public ConvertibleDerived(decimal amount)
+        {
+            Amount = amount;
+        }
+
+        public static implicit operator decimal(ConvertibleDerived derived) => derived.Amount;
+    }
+
+    public class PreciseQuantity : IComparable<PreciseQuantity>
+    {
+        public decimal Value { get; }
+
+        public PreciseQuantity(decimal value)
+        {
+            Value = value;
+        }
+
+        // equal to 1 once converted to double, only the type's own ordering tells them apart
+        public static PreciseQuantity AboveOne() => new(1.0000000000000000000000000001m);
+
+        public int CompareTo(PreciseQuantity other) => Value.CompareTo(other.Value);
+
+        public static implicit operator decimal(PreciseQuantity quantity) => quantity.Value;
+    }
+
+    public class NotConvertible
+    {
+    }
+
+    public class TestImplicitNumericOperators
+    {
+        private static dynamic module;
+
+        private const string TestModule = @"
+from clr import AddReference
+AddReference(""System"")
+AddReference(""Python.EmbeddingTest"")
+from Python.EmbeddingTest import *
+
+fee = ConvertibleFee(2)
+other = ConvertibleFee(3)
+
+def arithmetic():
+    return (fee + other, fee + 1, 1 + fee, fee - other, fee - 1, 10 - fee,
+        fee * other, fee * 2, 2 * fee, other / fee, fee / 2, 10 / fee, -fee)
+
+def aggregates():
+    return sum([fee, other]), max(fee, other) is other, sorted([other, fee])[0] is fee
+
+def comparisons():
+    return (fee < other, fee > other, fee <= other, fee >= other,
+        fee > 0, fee < 2, fee <= 2, fee >= 2, 1 < fee, 2 >= fee)
+
+def conversions():
+    return float(fee), int(ConvertibleRatio(1.75)), float(ConvertibleRatio(1.75))
+
+def own_operator_wins():
+    total = ConvertibleMoney(2) + ConvertibleMoney(3)
+    return type(total).__name__, total.Amount, ConvertibleMoney(2) - ConvertibleMoney(1)
+
+def not_convertible():
+    return hasattr(NotConvertible(), '__add__'), hasattr(NotConvertible(), '__float__')
+
+count = ConvertibleCount(2)
+big = ConvertibleCount(2**53 + 1)
+
+def integer_arithmetic():
+    results = (count + 1, 1 + count, count + count, count - 5, 10 - count, count * 3, 3 * count, -count, count + True)
+    return all(type(result) is int for result in results), results
+
+def integer_with_floats():
+    results = (count + 0.5, 0.5 + count, count * 1.5, count / 2, count / count, 3 / count)
+    return all(type(result) is float for result in results), results
+
+def large_integers():
+    exact = 2**53 + 1
+    return (int(big) == exact, big > 2**53, big - 1 == 2**53, big * big == exact * exact,
+        -big == -exact, big >= exact, big + 2**70 == exact + 2**70)
+
+def int_conversions():
+    errors = []
+    for value in (float('nan'), float('inf')):
+        try:
+            int(ConvertibleRatio(value))
+        except Exception as e:
+            errors.append(type(e).__name__)
+    return errors, int(ConvertibleRatio(-1.75)), int(ConvertibleFee(12345678901234567891)) == 12345678901234567891
+
+def division_by_zero():
+    errors = []
+    for operation in (lambda: fee / 0, lambda: 1 / ConvertibleFee(0), lambda: count / 0.0, lambda: fee / ConvertibleFee(0)):
+        try:
+            operation()
+        except ZeroDivisionError:
+            errors.append(True)
+    return len(errors)
+
+def other_numeric_operands():
+    margin = ConvertibleRatio(0.5)
+    return fee + 2**40, count + 2**70, fee + margin, count * margin, count < 2**64
+
+def unsupported_operands():
+    messages = []
+    for operation in (lambda: count + 'x', lambda: fee * None, lambda: fee < 'x', lambda: NotConvertible() - fee):
+        try:
+            operation()
+            messages.append(None)
+        except TypeError as e:
+            messages.append(str(e))
+    return messages
+
+def inherited_operator_wins():
+    derived = ConvertibleDerived(2)
+    return derived + derived, derived - ConvertibleDerived(1), float(derived)
+
+def own_ordering_wins():
+    one, above = PreciseQuantity(1), PreciseQuantity.AboveOne()
+    return above > one, one < above, above >= one, one <= above, above < one, sorted([above, one])[0] is one
+";
+
+        [OneTimeSetUp]
+        public void OneTimeSetUp()
+        {
+            PythonEngine.Initialize();
+            using var _ = Py.GIL();
+            module = PyModule.FromString("implicit_numeric_operators", TestModule);
+        }
+
+        [OneTimeTearDown]
+        public void Dispose()
+        {
+            PythonEngine.Shutdown();
+        }
+
+        [Test]
+        public void ArithmeticDelegatesToTheImplicitConversion()
+        {
+            using var _ = Py.GIL();
+            var result = module.arithmetic();
+            var expected = new[] { 5.0, 3.0, 3.0, -1.0, 1.0, 8.0, 6.0, 4.0, 4.0, 1.5, 1.0, 5.0, -2.0 };
+            for (var i = 0; i < expected.Length; i++)
+            {
+                Assert.AreEqual(expected[i], (double)result[i], $"index {i}");
+            }
+        }
+
+        [Test]
+        public void AggregatesWork()
+        {
+            using var _ = Py.GIL();
+            var result = module.aggregates();
+            Assert.AreEqual(5.0, (double)result[0]);
+            Assert.IsTrue((bool)result[1]);
+            Assert.IsTrue((bool)result[2]);
+        }
+
+        [Test]
+        public void ComparisonsDelegateToTheImplicitConversion()
+        {
+            using var _ = Py.GIL();
+            var result = module.comparisons();
+            var expected = new[] { true, false, true, false, true, false, true, true, true, true };
+            for (var i = 0; i < expected.Length; i++)
+            {
+                Assert.AreEqual(expected[i], (bool)result[i], $"index {i}");
+            }
+        }
+
+        [Test]
+        public void FloatAndIntConversions()
+        {
+            using var _ = Py.GIL();
+            var result = module.conversions();
+            Assert.AreEqual(2.0, (double)result[0]);
+            Assert.AreEqual(1, (int)result[1]);
+            Assert.AreEqual(1.75, (double)result[2]);
+        }
+
+        [Test]
+        public void TheTypesOwnOperatorsKeepWinning()
+        {
+            using var _ = Py.GIL();
+            var result = module.own_operator_wins();
+            Assert.AreEqual("ConvertibleMoney", (string)result[0]);
+            Assert.AreEqual(5m, (decimal)result[1]);
+            // subtraction is not declared by the type, so the synthesized one applies
+            Assert.AreEqual(1.0, (double)result[2]);
+        }
+
+        [Test]
+        public void IntegralConversionsBehaveLikeInts()
+        {
+            using var _ = Py.GIL();
+            var result = module.integer_arithmetic();
+            Assert.IsTrue((bool)result[0], "every result must be an int");
+            var expected = new long[] { 3, 3, 4, -3, 8, 6, 6, -2, 3 };
+            for (var i = 0; i < expected.Length; i++)
+            {
+                Assert.AreEqual(expected[i], (long)result[1][i], $"index {i}");
+            }
+        }
+
+        [Test]
+        public void IntegralConversionsGiveFloatsWherePythonWould()
+        {
+            using var _ = Py.GIL();
+            var result = module.integer_with_floats();
+            Assert.IsTrue((bool)result[0], "every result must be a float");
+            var expected = new[] { 2.5, 2.5, 3.0, 1.0, 1.0, 1.5 };
+            for (var i = 0; i < expected.Length; i++)
+            {
+                Assert.AreEqual(expected[i], (double)result[1][i], $"index {i}");
+            }
+        }
+
+        [Test]
+        public void LargeIntegralValuesStayExact()
+        {
+            using var _ = Py.GIL();
+            var result = module.large_integers();
+            for (var i = 0; i < 7; i++)
+            {
+                Assert.IsTrue((bool)result[i], $"index {i}");
+            }
+        }
+
+        [Test]
+        public void IntConversionFollowsPython()
+        {
+            using var _ = Py.GIL();
+            var result = module.int_conversions();
+            CollectionAssert.AreEqual(new[] { "ValueError", "OverflowError" }, ((PyObject)result[0]).As<string[]>());
+            Assert.AreEqual(-1, (int)result[1]);
+            Assert.IsTrue((bool)result[2], "decimal conversions must truncate exactly");
+        }
+
+        [Test]
+        public void DivisionByZeroRaisesLikePython()
+        {
+            using var _ = Py.GIL();
+            Assert.AreEqual(4, (int)module.division_by_zero());
+        }
+
+        [Test]
+        public void LargeIntsAndOtherNumbersAreOperands()
+        {
+            using var _ = Py.GIL();
+            var result = module.other_numeric_operands();
+            Assert.AreEqual(2.0 + Math.Pow(2, 40), (double)result[0]);
+            Assert.AreEqual("1180591620717411303426", result[1].ToString());
+            // another type with an implicit numeric conversion is read through its __float__
+            Assert.AreEqual(2.5, (double)result[2]);
+            Assert.AreEqual(1.0, (double)result[3]);
+            Assert.IsTrue((bool)result[4]);
+        }
+
+        [Test]
+        public void UnsupportedOperandsRaisePythonsTypeError()
+        {
+            using var _ = Py.GIL();
+            var messages = ((PyObject)module.unsupported_operands()).As<string[]>();
+            StringAssert.Contains("unsupported operand type(s) for +", messages[0]);
+            StringAssert.Contains("unsupported operand type(s) for *", messages[1]);
+            StringAssert.Contains("'<' not supported", messages[2]);
+            StringAssert.Contains("unsupported operand type(s) for -", messages[3]);
+        }
+
+        [Test]
+        public void InheritedOperatorsKeepWinning()
+        {
+            using var _ = Py.GIL();
+            var result = module.inherited_operator_wins();
+            Assert.AreEqual("base", (string)result[0]);
+            // subtraction is declared nowhere in the hierarchy, so the synthesized one applies
+            Assert.AreEqual(1.0, (double)result[1]);
+            Assert.AreEqual(2.0, (double)result[2]);
+        }
+
+        [Test]
+        public void TheTypesOwnOrderingWinsBetweenItsInstances()
+        {
+            using var _ = Py.GIL();
+            var result = module.own_ordering_wins();
+            var expected = new[] { true, true, true, true, false, true };
+            for (var i = 0; i < expected.Length; i++)
+            {
+                Assert.AreEqual(expected[i], (bool)result[i], $"index {i}");
+            }
+        }
+
+        [Test]
+        public void TypesWithoutAConversionAreUntouched()
+        {
+            using var _ = Py.GIL();
+            var result = module.not_convertible();
+            Assert.IsFalse((bool)result[0]);
+            Assert.IsFalse((bool)result[1]);
+        }
+    }
+}
@@ -415,6 +415,16 @@ private static ClassInfo GetClassInfo(Type type, ClassBase impl)
                     info = info.Concat(opsImpl.GetMethods(OpsHelper.BindingFlags)).ToArray();
                 }
             }
+            else if (ImplicitNumericOps.IsSupported(type))
+            {
+                // Types with an implicit numeric conversion get Python's number protocol, keeping the operators they declare or inherit
+                var ops = ImplicitNumericOps.GetMethods(type);
+                foreach (var op in ops)
+                {
+                    local.Add(op.Name);
+                }
+                info = info.Concat(ops).ToArray();
+            }
 
             // Extension methods are bound as members so they can be called from the instance. A real member with the same name wins, as in C#
             var extensionMethods = ExtensionMethods.GetMethodsToBind(type).Where(em => !originalMemberNames.Contains(em.Name)).ToArray();