Type inference doesn't perform enough subtyping

The following does not work:
output.dummy(
  fallback([strip_blank(sine()), sine()]),
  fallible=true)
I get:
  this value has type
    source(_)
  but it should be a subtype of (the type of the value at line 1, char 35-42)
    active_source(_)

It is my understanding that there should be no technical reasons to prohibit this and so there must be something to do at the type-checking level..

Edit: The problem is that type inference guesses that this is a list of active_sources, but the second one isn't active; we need a constraint-based type inference algorithm to integrate subtyping in smarter ways, eg. delay the guess until we've seen all elements.