fix(result): Result<T> hash collision

A success result wrapping a value whose GetHashCode is 0 (e.g. Result<int>
with 0) collides with a failure result, both hashing to 0.

CHANGELOG.md

@@ -48,6 +48,7 @@ To have a consistent experience across all packages, some public interfaces have
 ### Fixed
 
 - **request.result:** Correct the namespace in the `Prelude` doc comment (`Geekeey.Extensions.Result` → `Geekeey.Request.Result`)
+- **request.result:** Fold `IsSuccess` into `Result<T>.GetHashCode` to avoid collisions between a failure and a success value whose hash is `0`
 
 ### Removed
 

src/request.result.tests/ResultEqualityTests.cs

@@ -166,11 +166,28 @@ internal sealed class ResultEqualityTests
 	}
 
 	[Test]
-	public async Task I_can_get_hashcode_and_get_zero_for_failure()
+	public async Task I_can_get_hashcode_and_get_non_zero_for_failure()
 	{
 		var result = Prelude.Failure<int>("error");
 
-		await Assert.That(result.GetHashCode()).IsZero();
+		await Assert.That(result.GetHashCode()).IsNotEqualTo(0);
+	}
+
+	[Test]
+	public async Task I_can_get_hashcode_and_not_collide_non_generic_success_and_failure()
+	{
+		await Assert.That(Prelude.Success().GetHashCode())
+			.IsNotEqualTo(Prelude.Failure("error").GetHashCode());
+	}
+
+	[Test]
+	public async Task I_can_get_hashcode_and_not_collide_success_value_zero_with_failure()
+	{
+		var success = Prelude.Success(0);
+		var failure = Prelude.Failure<int>("error");
+
+		await Assert.That(success.Equals(failure)).IsFalse();
+		await Assert.That(success.GetHashCode()).IsNotEqualTo(failure.GetHashCode());
 	}
 
 	[Test]
@@ -199,4 +216,53 @@ internal sealed class ResultEqualityTests
 
 		await Assert.That(a.Equals(b)).IsFalse();
 	}
+
+	[Test]
+	public async Task I_can_use_equality_operator_and_get_false_for_success_and_failure()
+	{
+		var success = Prelude.Success(2);
+		var failure = Prelude.Failure<int>("error");
+
+		await Assert.That(success == failure).IsFalse();
+		await Assert.That(success != failure).IsTrue();
+	}
+
+	[Test]
+	public async Task I_can_use_equality_operator_and_get_true_for_successes_with_equal_value()
+	{
+		await Assert.That(Prelude.Success(2) == Prelude.Success(2)).IsTrue();
+		await Assert.That(Prelude.Success(2) != Prelude.Success(3)).IsTrue();
+	}
+
+	[Test]
+	public async Task I_can_use_equality_operator_and_compare_result_to_value()
+	{
+		await Assert.That(Prelude.Success(2) == 2).IsTrue();
+		await Assert.That(Prelude.Success(2) != 3).IsTrue();
+		await Assert.That(Prelude.Failure<int>("x") == 2).IsFalse();
+	}
+
+	[Test]
+	public async Task I_can_equal_object_and_get_false_for_null()
+	{
+		object result = Prelude.Success(2);
+
+		await Assert.That(result.Equals(null)).IsFalse();
+	}
+
+	[Test]
+	public async Task I_can_equal_object_and_get_false_for_wrong_type()
+	{
+		object result = Prelude.Success(2);
+
+		await Assert.That(result.Equals("not a result")).IsFalse();
+	}
+
+	[Test]
+	public async Task I_can_equal_object_and_get_true_for_boxed_equal_result()
+	{
+		object result = Prelude.Success(2);
+
+		await Assert.That(result.Equals(Prelude.Success(2))).IsTrue();
+	}
 }

src/request.result/Result.Equality.cs

@@ -181,7 +181,12 @@ public partial class Result<T> : IEquatable<Result<T>>, IEquatable<T>
 			return comparer.GetHashCode(result.Value);
 		}
 
-		return 0;
+		if (result is { IsSuccess: true, Value: null })
+		{
+			return 0;
+		}
+
+		return result.Error?.GetHashCode() ?? 0;
 	}
 }