On Sunday, 16 June 2019 at 15:11:29 UTC, Robert M. Münch wrote:
How does the observerObject Template and function work? I'm struggling because both use the same name and how is the template parameter R deduced/where is it coming from? Looks like it's somehow implicitly deduced.

Eponymous templates:
https://dlang.org/spec/template.html#implicit_template_properties

"Templated types" are actually particular cases of eponymous templates:
https://dlang.org/spec/template.html#StructTemplateDeclaration

   class ObserverObject(R, E...) {...}

is equivalent to

   tempalte ObserverObject(R, E...)
   {
      class ObserverObject(R, E...) {...}
   }

So this is I think how everything is made to work with the same compiler engine, both individual "templated types" and "eponymous templates".

It's considered idiomatic, but if you don't like it in your case, it's very easy for the author to avoid it: just make the names different in any way.

   template Observer(E)
   {
      ObserverObject!(R, E) Object(R)(R range)
      {
          return new ObserverObject!(R, E)(range);
      }
   }

   auto observer = Observer!int.Object(TestObserver());

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