[Issue 23554] New: Can break immutable with delegate

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23554

          Issue ID: 23554
           Summary: Can break immutable with delegate
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: andy.pj.hanson at gmail.com

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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