Typechecker fails to match non-primitive return types inside tuple on function types

[Bearmarshal]

def fun() : (String, String) {
  ("413", "612")
}

class Main {
  def main() : void {
    let foo = fun : () -> (String, String);
  }
}

The above error fails compilation with the following error on the development branch:

Type '() -> (String, String)' does not match expected type '() -> (String, String)'
In expression: 
  fun
In expression: 
  fun : () -> (String, String)
In expression: 
  let foo = fun : () -> (String, String)
In expression: 
  {let foo = fun : () -> (String, String)}
In method 'main' of type 'void'
In class 'Main'

The error triggers with any non-primitive types inside the tuple. It does not happen with a non-tuple return type, or a tuple containing only primitives.