`checkGrammarTypeInfer` validation ignores declared types in other .langium files

Definition of a rule X and a type XType in different files, gives you a validation error The type 'XType' is not explicitly declared and must be inferred.

interface XType {
    a: string
}

X returns XType:
    a=ID;

[dhuebner]

@spoenemann The unresolved return type check you added works as expected and there is no other much better place where to put it.
So I would propose to close this issue. We can create a new one if the problem described here occurs also after the type system refactoring.

[dhuebner]

@pluralia I can’t reproduce this issue using the example you provided. Could you add a real world example that produces the error mentioned above?