Usually static type inference does not cause an invalid program to type check. But what it can do is give a static type error that’s pretty far removed from the “real” source of the problem as seen by the programmer. Scala requires type annotations on arguments to functions which is a bummer but on the plus side you don’t tend to have that kind of chained inference problems. In languages with fuller type inference (Haskell, ML, F#) it’s common practice to annotate types on many or most top level definitions just to prevent that kind of problem.

Scala’s compulsory type annotations on function arguments has to do with the limitations on currently known inference algorithms and how they relate to subtyping and overloading. F# also has some, though far fewer, spots where it needs explicit type annotations.