A simpler version of the problem:

   jshell> int x = x + 3
   x ==> 3

Which should give an error.

------------

FULL PRODUCT VERSION :
java version "10" 2018-03-20
Java(TM) SE Runtime Environment 18.3 (build 10+46)
Java HotSpot(TM) 64-Bit Server VM 18.3 (build 10+46, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
macOS High Sierra version 10.13.1 

A DESCRIPTION OF THE PROBLEM :

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I was working on a problem to store reference of two classes within each other For Example:

class A {
B b;
A(B b){
this.b = b;}
}

class B {
A a;
B(A a){
this.a = a;}
}

public static void main(String...s){
A a = new A(new B(null));
a.b.a = a;
}
Now if instead of above initialisation, if I use below statement:

A a = new A(new B(a));
I got below error which is quite obvious:

Main.java:19: error: variable a might not have been initialised
        A a = new A(new B(a));

But if I try the same on JShell, it works just fine (Just to be extra sure that variable a has never been initialized, I checked for variable a just before executing the statement which confirms that it was not initialized before:

jshell> a
|  Error:
|  cannot find symbol
|    symbol:   variable a
|  a
|  ^

jshell> A a = new A(new B(a));
a ==> A@57fffcd7

jshell> a.b.a = a
$4 ==> A@57fffcd7

jshell> a.b.a
$5 ==> A@57fffcd7

jshell> a
a ==> A@57fffcd7

 There are two different behaviours of the same statement being executed in JAVA using standard compiling process and JShell





STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Step 1: Start JShell
Step 2: Execute below code:
jshell> class A { 
   ...> B b;
   ...> A(B b) {
   ...> this.b =b;
   ...> }
   ...> }
|  created class A, however, it cannot be referenced until class B is declared

jshell> class B {
   ...> A a;
   ...> B(A a){
   ...> this.a = a;
   ...> }
   ...> }
|  created class B

jshell> A a = new A(new B(a));
a ==> A@57fffcd7


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
error: variable a might not have been initialised
        A a = new A(new B(a));
ACTUAL -
Instance created

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
class A {
B b;
A(B b){
this.b = b;}
}

class B {
A a;
B(A a){
this.a = a;}
}

A a = new A(new B(a));
---------- END SOURCE ----------