Allow building from a detached tag, since there is no need to calculate the increment

src/GitVersionCore.Tests/IntegrationTests/TagCheckoutScenarios.cs

@@ -0,0 +1,32 @@
+using GitTools.Testing;
+using NUnit.Framework;
+
+namespace GitVersionCore.Tests.IntegrationTests
+{
+    [TestFixture]
+    public class TagCheckoutScenarios
+    {
+        [Test]
+        public void GivenARepositoryWithSingleCommit()
+        {
+            using var fixture = new EmptyRepositoryFixture();
+            const string taggedVersion = "1.0.3";
+            fixture.Repository.MakeATaggedCommit(taggedVersion);
+            fixture.Checkout(taggedVersion);
+
+            fixture.AssertFullSemver(taggedVersion);
+        }
+
+        [Test]
+        public void GivenARepositoryWithSingleCommitAndSingleBranch()
+        {
+            using var fixture = new EmptyRepositoryFixture();
+            const string taggedVersion = "1.0.3";
+            fixture.Repository.MakeATaggedCommit(taggedVersion);
+            fixture.BranchTo("task1");
+            fixture.Checkout(taggedVersion);
+
+            fixture.AssertFullSemver(taggedVersion);
+        }
+    }
+}

src/GitVersionCore/VersionCalculation/NextVersionCalculator.cs

@@ -35,7 +35,10 @@ public SemanticVersion FindVersion()
                 log.Info($"Current commit is tagged with version {context.CurrentCommitTaggedVersion}, " +
                          "version calculation is for metadata only.");
             }
-            EnsureHeadIsNotDetached(context);
+            else
+            {
+                EnsureHeadIsNotDetached(context);
+            }
 
             SemanticVersion taggedSemanticVersion = null;