More on purity

[bearophile]

This was discussed a little in D.learn. Is code like this going to compile in 
some future DMD versions, like DMD 2.058? I hit situations like this often in 
my code:



int[] foo1(immutable int[] a) pure {
    return new int[a.length];
}

int[] foo2(in int[] a) pure {
    return new int[a.length];
}

void main() {
    immutable int[] arr1 = [1, 2, 3];
    immutable int[] r1 = foo1(arr1); // OK

    const int[] arr2 = [1, 2, 3];
    immutable int[] r2 = foo2(arr2); // error
}


Currently it gives:
Error: cannot implicitly convert expression (foo2(arr2)) of type int[] to 
immutable(int[])


If code like that can't be accepted in future D/DMD versions, then I think DMD 
should give an error message that explains why the r2 implicitly conversion is 
not acceptable (while the implicitly conversion to r1  is acceptable).

Bye,
bearophile