[Issue 24552] New: cast to immutable in @safe not rejected

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24552

          Issue ID: 24552
           Summary: cast to immutable in @safe not rejected
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: accepts-invalid
          Severity: critical
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: elpenguino+D at gmail.com

```
void fun() @safe {
    static struct Foo(E) {
        private E[] data;
        Foo!NewElement opCast(T : Foo!NewElement, NewElement)() {
            return Foo!NewElement(cast(NewElement[])data);
        }
    }
    int[] data = [1,2,3,4,5];
    Foo!int tmp = Foo!int(data);
    immutable tmp2 = (cast(immutable Foo!uint)tmp).data;
    assert(tmp2[2] == 3);
    data[2] = 77;
    assert(tmp2[2] == 77); // immutable data mutated?!
}
```
This should not be allowed to compile.

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