[Issue 23297] New: You Can Assign a dstring to a dchar[] if Both Sides of the Expression are Slices

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23297

          Issue ID: 23297
           Summary: You Can Assign a dstring to a dchar[] if Both Sides of
                    the Expression are Slices
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: rubytheroobster at yandex.com

The following code fails, like it should:

```d
void main()
{
    dchar[] a; //asset(typeof(a).stringof == "dchar[]")
    dstring b = "a"d; //assert(typeof(b).stringof == "immutable(dchar)[]");
    a = b; //ERROR: VIOLATES THE TYPE SYSTEM
}
```

However, the following code compiles and runs:

```d
void main()
{
    import std.stdio;
    dchar[] a = "ab".dup;
    writeln(typeof(a[0 .. $-1]).stringof); //Prints 'dchar[]'
    writeln(typeof(a[1 .. $]).stringof); //Prints 'immutable(dchar)[]'
    a[0 .. $-1] = a[1 .. $].idup; //Compiles, but WRONG
    --a.length;
    writeln(a); //Prints 'b' to stdout
}
```

Given that the first example fails, the second example should also fail and
output a similar error.

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