Whats going on with this?

[claptrap]

---
import std;

import std.stdio;

struct Foo
{
     int a = 0, b = 0;

     this(int[2] vars)
     {
         this.a = vars[0];
         this.b = vars[1];
     //    writeln("constructor called");
     }

}

Foo foo = [300,300];

void main()
{
     writeln(foo.a);
}
---

Compiles and works OK. I cant see anything in the struct docs 
explaining why that array on the right hand side is automatically 
converted to a constructor call.

Weird thing is if you un-comment the writeln in the constructor 
it wont compile. You get this...

/dlang/dmd/linux/bin64/../../src/phobos/std/stdio.d(4839): Error: 
variable impl cannot be modified at compile time
/dlang/dmd/linux/bin64/../../src/phobos/std/stdio.d(3806):        
called from here: makeGlobal()
/dlang/dmd/linux/bin64/../../src/phobos/std/stdio.d(3896):        
called from here: trustedStdout()
onlineapp.d(13):        called from here: writeln("constructor 
called")
onlineapp.d(18):        called from here: Foo(0, 0).this([300, 
300])

How ever if you move Point foo declaration into the main function 
it works OK.

What exactly is going on? I mean it looks like the Foo is default 
constructed and then constructed on top with a call to the arary 
constructor? But why?

Why would putting in the writeln cause it to fail? Is it maybe 
trying to create the foo at compile time?