The following allows a write to an immutable array:

import std.stdio;

void doSomething(const(char)[][] a, const(char)[][] b)
{
    a[0]=b[0];
}

void main()
{
    string s = "hello";
    char[][1] a;
    string[1] b;
    b[0] = s;
    doSomething(a, b);
    a[0][1] = 'c';   // OOPS!
    writeln(s);
}

So, a conversion to const that is not the top ref should be disallowed.