little bit of input validation on projection registration to prevent …

…confusion on Snapshot registrations

src/EventSourcingTests/Projections/SingleStreamProjectionTests.cs

@@ -1,3 +1,4 @@
+using System;
 using EventSourcingTests.Aggregation;
 using Marten;
 using Marten.Events;
@@ -41,4 +42,15 @@ public void do_not_set_mapping_to_UseVersionFromMatchingStream_when_rich_append(
 
         mapping.UseVersionFromMatchingStream.ShouldBeFalse();
     }
+
+    [Fact]
+    public void you_cannot_accidentally_try_to_add_single_stream_projection_through_the_snapshot()
+    {
+        var options = new StoreOptions();
+
+        Should.Throw<InvalidOperationException>(() =>
+        {
+            options.Projections.Snapshot<AllGood>(SnapshotLifecycle.Inline);
+        });
+    }
 }

src/Marten/Events/Projections/ProjectionOptions.cs

@@ -324,6 +324,10 @@ private MartenRegistry.DocumentMappingExpression<T> singleStreamProjection<T>(
         Action<AsyncOptions> asyncConfiguration = null
     )
     {
+        if (typeof(T).CanBeCastTo<IProjectionSource>())
+            throw new InvalidOperationException(
+                $"This registration mechanism can only be used for an aggregate type that is 'self-aggregating'. Please use the Projections.Add() API instead to register {typeof(T).FullNameInCode()}");
+
         // Make sure there's a DocumentMapping for the aggregate
         var expression = _options.Schema.For<T>();