A matter of inout

[bearophile]

This program used to compile in DMD 2.059:

import std.stdio;
inout(T[]) forwardDifference(T)(inout(T[]) s, in int n) pure {
    foreach (_; 0 .. n)
        s[] -= s[1 .. $];
    return s[0 .. $-n];
}
void main() {
    immutable A = [90.5, 47, 58, 29, 22, 32, 55, 5, 55, 73.5];
    foreach (level; 0 .. A.length)
        writeln(forwardDifference(A.dup, level));
}


Now it gives:

test.d(5): Error: slice s[] is not mutable
test.d(12): Error: template instance 
test.forwardDifference!(double) error instantiating

Is it a 2.060 regression (or it's caused by a bug fix)?

Bye,
bearophile