I've made the example in Java but I think (not tested) that it works in other (all?) languages.

You have 2 files M.java that says

public class MType{
    XType x;
   MType(){  x = null;}
 }

and another file XType.java (in the same directory)

public class XType{

   MType m;

   public XType(MType m){  this.m=m;}
}

Ok it's BAD programming , but.. if you run javac XType it compiles: compiles even MTypes because XType needs it. But.. MType needs XType.. how it works? How does the compiler know what is happening?
Probably is a stupid question, but I would like to know how the compiler (javac or other compilers if you know.) manages that situation, not how to avoid it.
I'm asking because i'm writing a precompiler and I would like to manage that situation..
Thank you