Bug 23554 – Can break immutable with delegate

This code is able to mutate an `immutable int[][]`: ``` @safe: import std.stdio : writeln; void main() { immutable int[][] xs = [[1]]; writeln("xs[0][0] is ", xs[0][0]); // 1 f(xs); writeln("xs[0][0] is ", xs[0][0]); // 2 } void f(immutable int[][] xs) { each(xs, (ref immutable(int)[] x) { x = [2]; }); } void each(immutable int[][] xs, void delegate(ref immutable int[]) @safe pure nothrow cb) { foreach (ref x; xs) cb(x); } ``` The provided delegate `ref immutable(int)[] x` should not be allowed, since it should take a `ref immutable int[]` instead.

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