import std.traits : CopyConstness;

static struct S

{

   int[int] aa;

   alias aa this;

}

​

auto foo(T: V[K], V, K)(T t)

{

    CopyConstness!(T, V[K]) aa = t;

    return aa;

}

​

void main()

{

    const(int[int]) x;

    auto aa1 = foo(x);

    static assert(is(typeof(aa1) == const(int[int])));

​

    const(S) s;

    auto aa2 = foo(s);

    static assert(is(typeof(aa2) == const(int[int])));

}