Surprise with array idup method

[Philip Daniels]

auto x = [1,2,3];
auto y = x.idup;
y ~= 99;   // fine!
y[0] = 99; // "Error: y[0] isn't mutable"
y.clear;   // fine!

So idup is returning an "immutable(int)[]" rather than an 
"immutable int[]".

I find this a bit surprising. Anybody else?