immutable int n = Test(); int[n] x; ---- compiles, but __ctfe is false. How?

[Gopan]

Attempting to learn CTFE, I tried the following test.
size_t counter;

uint Test()
{
        if (!__ctfe)
        {
                ++counter;// This code is for execution at run time
        }
        return 2;
}

void main()
{
        writeln("counter = ", counter);
        
        immutable int n = Test();
        int[n] arr;
	writeln("arrary length = ", arr.length, " ; counter = ", 
counter);
}

output:
counter = 0
arrary length = 2 ; counter = 1

For array declaration to be successful, its size has to be known 
at compile time.  The above code compiles too.  But __ctfe seems 
to be false while performing Test().

Instead, if I write
        int[Test()] c;
        writeln("c.length = ", c.length, " ; counter = ", counter);
output is
counter = 0
c.length = 2 ; counter = 0

What is wrong in my mind?

Thanks,
Gopan