Consider the following code:
object SomeObjectA {
object SomeObjectB {
val a = "test"
}
}
val X = SomeObjectA
typealias Y = SomeObjectA
SomeObjectA.SomeObjectB // works
X.SomeObjectB // error
Y.SomeObjectB // error
I can't use val or typealias, which reference the outer object, to refer to a nested object within that outer object. Why?
the compiler error is comes from java, and the kotlin objects convert to java classes as below:
public final class SomeObjectA {
private SomeObjectA() {/**/}
public static final SomeObjectA INSTANCE = new SomeObjectA();
public static final class SomeObjectB {
private SomeObjectB() {/**/}
public static final SomeObjectB INSTANCE = new SomeObjectB();
}
}
SomeObjectA.SomeObjectB is compiled to java code as below:
SomeObjectA.SomeObjectB.INSTANCE;
SomeObjectA is compiled to java code as below:
SomeObjectA.INSTANCE
we know kotlin is base on java, and java don't allow access the nested classes via instance reference, if you do the compiler will reports an error:"Error: java: unexpected type required: class,package found: variable", for example:
SomeObjectA a = SomeObjectA.INSTANCE;
SomeObjectB b = a.SomeObjectB.INSTANCE;// error
// ^--- compiler don't know where to go? package&class or variable?
the code below, kotlin compiler will transforms the java compiler error as: "Error: Kotlin: Nested object 'SomeObjectB' accessed via instance reference".
val a = SomeObjectA;
val b = a.SomeObjectB;
// ^--- Error
Type aliases do not introduce new types. They are equivalent to the corresponding underlying types.
so the two statements below are equality:
val a = SomeObjectA;
typealias a2 = SomeObjectA;
avoiding to the use of typealias causing unnecessary compiler error, kotlin doesn't include all nested classes in typealias.
What you described happens because SomeObjectA in your example is simultaneously a name of an object and the name of its class.
So to access SomeObjectB, you need to use the <classname>.<classname> syntax. That is why X.SomeObjectB doesn't compile (<object>.<classname> is unsupported)
P.S. This doesn't really explain your second problem with typealias. It looks like like a bug to me, but I'm not sure.
Y.SomeObjectB, reported as KT-24902, since other issues are similar, but not really the same.