Fix computing next Depth from parentIteration

src/GitVersion.Core.Tests/VersionCalculation/MainlineIterationTests.cs

@@ -0,0 +1,19 @@
+using GitVersion.Configuration;
+using GitVersion.VersionCalculation.Mainline;
+
+namespace GitVersion.Core.Tests.VersionCalculation;
+
+[TestFixture]
+public class MainlineIterationTests
+{
+    [Test]
+    public void ChildIsDeeperThanParent()
+    {
+        var configuration = new BranchConfiguration();
+        var parent = new MainlineIteration("id0", new("canonical0"), configuration, null, null);
+        var child = new MainlineIteration("id1", new("canonical1"), configuration, parent, null);
+
+        parent.Depth.ShouldBe(1);
+        child.Depth.ShouldBe(2);
+    }
+}

src/GitVersion.Core/VersionCalculation/Mainline/MainlineIteration.cs

@@ -55,7 +55,7 @@ public MainlineIteration(string id, ReferenceName branchName, IBranchConfigurati
         MainlineIteration? parentIteration, MainlineCommit? parentCommit)
     {
         Id = id.NotNullOrEmpty();
-        Depth = parentIteration?.Depth ?? 0 + 1;
+        Depth = (parentIteration?.Depth ?? 0) + 1;
         BranchName = branchName.NotNull();
         Configuration = configuration.NotNull();
         ParentIteration = parentIteration;