Reference and pointer types (annotated with `ref` and `ptr` respectively) are nillable. The compiler can provide null safety guarantees by annotating types with

Reference and pointer types (annotated with `ref` and `ptr` respectively) are nillable. The compiler can provide null safety guarantees by annotating types with `not nil`: https://nim-lang.github.io/Nim/manual_experimental.html#not-nil-annotation https://nim-lang.github.io/Nim/manual_experimental.html#not-.... So then your example would look like the following, and fail to compile. type Foo = ref object Bar = Foo not nil var bar: Bar = nil # error: type mismatch echo bar.a Discussion about not nil by default is here: https://github.com/nim-lang/Nim/issues/6638 https://github.com/nim-lang/Nim/issues/6638