Elm modules are currently permitted to expose values whose type signatures mention types that the module does not expose, making the types of these public values "unmentionable" by code in any other modules. For example:

module ModuleA exposing (public)

type Private = Private

public : Private
public = Private

module ModuleB exposing (..)

import ModuleA

-- Can't give `foo` a type signature because `ModuleA.Private` is not exposed
foo = ModuleA.public

Making this at least a warning would prevent two possible classes of mistakes. First of all, it would prevent the above situation of a module author forgetting to expose a type that's essential to using its public values with explicit type annotations. Second of all, it would prevent module authors from thinking they've kept a type entirely private, when in fact it's partially exposed through a public type signature.