Prevent issues for those who wish use ServiceBasedControllerActivator.

Adds registration for CurrentUserController which has ambiguous constructors.
Register our controllers so that issues are visible during dev/test.

src/Umbraco.Web.UI/Composers/ControllersAsServicesComposer.cs

@@ -0,0 +1,61 @@
+using System;
+using System.Linq;
+using Microsoft.AspNetCore.Mvc.Controllers;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.DependencyInjection.Extensions;
+using Umbraco.Cms.Core.Composing;
+using Umbraco.Cms.Core.DependencyInjection;
+
+namespace Umbraco.Cms.Web.UI.Composers
+{
+    /// <summary>
+    /// Adds controllers to the service collection.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// Umbraco 9 out of the box, makes use of <see cref="DefaultControllerActivator"/> which doesn't resolve controller
+    /// instances from the IOC container, instead it resolves the required dependencies of the controller and constructs an instance
+    /// of the controller.
+    /// </para>
+    /// <para>
+    /// Some users may wish to switch to <see cref="ServiceBasedControllerActivator"/> (perhaps to make use of interception/decoration).
+    /// </para>
+    /// <para>
+    /// This composer exists to help us detect ambiguous constructors in the CMS such that we do not cause unnecessary effort downstream.
+    /// </para>
+    /// <para>
+    /// This Composer is not shipped by the Umbraco.Templates package.
+    /// </para>
+    /// </remarks>
+    public class ControllersAsServicesComposer : IComposer
+    {
+        /// <inheritdoc />
+        public void Compose(IUmbracoBuilder builder) => builder.Services
+            .AddMvc()
+            .AddControllersAsServicesWithoutChangingActivator();
+    }
+
+    internal static class MvcBuilderExtensions
+    {
+        /// <summary>
+        /// <see cref="MvcCoreMvcBuilderExtensions.AddControllersAsServices"/> but without the replacement of
+        /// <see cref="DefaultControllerActivator"/>.
+        /// </summary>
+        /// <remarks>
+        /// We don't need to opt in to <see cref="ServiceBasedControllerActivator"/> to ensure container validation
+        /// passes.
+        /// </remarks>
+        public static IMvcBuilder AddControllersAsServicesWithoutChangingActivator(this IMvcBuilder builder)
+        {
+            var feature = new ControllerFeature();
+            builder.PartManager.PopulateFeature(feature);
+
+            foreach (Type controller in feature.Controllers.Select(c => c.AsType()))
+            {
+                builder.Services.TryAddTransient(controller, controller);
+            }
+
+            return builder;
+        }
+    }
+}