Implicit conversion in constructor

[rcorre via Digitalmars-d-learn]

Is there any reason why implicit conversion from Foo to Thing is 
permitted in a regular method but not in a constructor?

Trying to figure out whether this is a bug or some sort of 
constructor-specific safety precaution.

struct Thing {
   Foo foo;
   alias foo this;
}

class Foo { }

class Bar {
   Thing thing;

   void fine() {
     thing = new Foo(); // ok
   }

   this() {
     thing = new Foo(); // nope!
   }
}