bug.ts

type GraphQLType =
    | GraphQLInt
    | GraphQLList<any>
    | GraphQLNonNull<any>;

interface GraphQLInt {
  int: number
}

interface GraphQLList<T> {
  x: string
}

interface GraphQLNonNull<T> {
  x: string
}

declare function isInt(type: GraphQLType): type is GraphQLInt;
declare function isList(type: GraphQLType): type is GraphQLList<any>;
declare function isNonNull(type: GraphQLType): type is GraphQLNonNull<any>;

function doIt(t: GraphQLType) {
  if (isInt(t)) {
    return t.int
  }

  if (isList(t)) {
    return t.x
  }

  if (isNonNull(t)) {
    return t.x
  }
}