Those functional languages don't have function overloading or named arguments. The type inference in Crystal works very differently. The type of a method can only be computed from explicit calls to it. In those functional languages the type can be computed independent of a call because it's trivial to do so if a function name always refers to a single entity.
> Those functional languages don't have function overloading or named arguments.
Scala has both, F# too (though function overloading is possible, I think it's not idiomatic), OCaml has named arguments, Haskell has overloading.
Compared to Haskell, Ocaml, Scala, F#, ...? Writing it like a dynamically typed language is standard for full type inference.